Synergistic Effects of Alumina and Graphite Reinforcement on Microstructural Evolution and Tribological Performance of Friction Stir Processed Copper based Composites

Synergistic Effects of Alumina and Graphite Reinforcement on Microstructural Evolution and Tribological Performance of Friction Stir Processed Copper based Composites | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Synergistic Effects of Alumina and Graphite Reinforcement on Microstructural Evolution and Tribological Performance of Friction Stir Processed Copper based Composites Ahmadreza Farjood, Mostafa jafarzadegan, Reza Taghiabadi This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8488460/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 12 You are reading this latest preprint version Abstract Hybrid metal matrix composites of copper reinforced with varying proportions of alumina and graphite particles were fabricated through two-pass friction stir processing. Five compositions spanning 100% Al₂O₃ to 100% Gr were investigated through microstructural, mechanical, and tribological characterization. Dynamic recrystallization generated refined equiaxed grain structures with progressive refinement as graphite content increased, attributed to enhanced Zener pinning. Vickers hardness measurements revealed compositional dependence, with pure graphite achieving maximum hardness of approximately 140 HV—surpassing pure alumina—demonstrating that grain refinement through self-lubricating particle pinning equals ceramic-based strengthening. Pin-on-disc testing documented 80% wear reduction in pure graphite relative to unprocessed copper. The 50Al₂O₃-50Gr balanced composition achieved optimal performance through synergistic effects: near-maximum hardness, 70% wear reduction, and significantly reduced friction coefficients. Wear mechanism analysis established progressive transitions from abrasive wear in alumina samples toward boundary lubrication in graphite-enriched materials, confirmed through wear surface morphology and debris characterization. The investigation demonstrates that carefully engineered hybrid reinforcement proportions deliver superior tribological performance through synergistic interactions between ceramic strengthening and graphite lubrication, providing guidance for designing advanced copper-based composites for bearing and wear-resistant applications. Physical sciences/Engineering Physical sciences/Materials science Friction stir processing Hybrid composites Alumina-graphite reinforcement Tribological performance Microstructural refinement Wear resistance Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Figure 12 Figure 13 Figure 14 1. Introduction The quest for advanced materials with superior tribological characteristics has become increasingly critical in modern engineering applications, particularly where friction and wear resistance directly influence component longevity and operational efficiency. Copper and its alloys have long been recognized as promising candidates for bearing systems, electrical contacts, and wear-resistant applications owing to their excellent thermal conductivity, electrical properties, and inherent workability. However, unmodified copper matrices demonstrate inherent limitations in mechanical strength and wear resistance when subjected to demanding operational conditions, necessitating strategies to enhance their performance through material modification and reinforcement mechanisms [ 1 , 2 ]. Over the past two decades, the field of surface engineering has witnessed remarkable progress through the development of metal matrix composites (MMCs), which strategically incorporate reinforcing particles into metallic matrices to achieve synergistic combinations of properties unattainable by either constituent alone. Among the various manufacturing techniques available, friction stir processing (FSP) has emerged as a particularly promising approach for fabricating high-quality surface composites with refined microstructures and enhanced mechanical attributes. Unlike conventional casting-based methods that often suffer from particle segregation and porosity-related defects, FSP operates through a solid-state mechanism wherein a rotating tool with specially designed geometry generates intense localized plastic deformation and dynamic recrystallization within the processed zone, creating an environment conducive to uniform particle distribution and robust interfacial bonding between reinforcement phases and the base matrix [ 3 – 5 ]. The fundamental appeal of FSP lies in its ability to harness dynamic recrystallization (DRX) mechanisms—including continuous dynamic recrystallization (CDRX) and geometric dynamic recrystallization (GDRX)—which collectively facilitate the formation of remarkably fine and equiaxed grain structures within the stir zone. These refined microstructures are instrumental in elevating hardness values and mechanical strength through the well-established Hall-Petch relationship, whereby grain size refinement directly translates into enhanced dislocation pile-up resistance and improved load-bearing capacity. Simultaneously, the mechanical mixing action of the rotating tool ensures homogeneous spatial distribution of reinforcing particles throughout the processed layer, eliminating problematic agglomeration zones that typically compromise composite performance in conventionally manufactured specimens [ 6 – 8 ]. The selection of appropriate reinforcing phases constitutes a critical consideration in composite design philosophy. Alumina (Al₂O₃) has demonstrated consistent effectiveness as a reinforcement phase, offering exceptional hardness, chemical stability, and load-bearing characteristics that significantly enhance the strength and wear resistance of metallic matrices. Its ceramic nature and high elastic modulus enable effective stress transfer mechanisms from the ductile copper matrix to the rigid ceramic particles, resulting in measurable improvements in hardness and tensile properties. However, the pursuit of exclusively enhanced mechanical strength through ceramic reinforcement often comes at the expense of tribological performance, particularly regarding friction coefficient reduction and self-lubricating capacity [ 9 , 10 ]. Graphite (Gr), conversely, introduces fundamentally different material characteristics that complement the properties provided by alumina reinforcement. As a layered carbon-based solid lubricant with exceptional anisotropic properties, graphite exhibits natural self-lubricating tendencies stemming from the weak van der Waals bonding between its basal planes. When graphite particles are dispersed within a metallic matrix, they facilitate the formation of protective transfer films during sliding contact, effectively reducing the friction coefficient and moderating wear rates through mechanisms that operate distinctly differently from those associated with hard ceramic reinforcements. The inherent lubricity of graphite, combined with its capacity to generate mechanically mixed layers (MML) during tribological engagement, creates a synergistic effect wherein ceramic hardness and solid-state lubrication act in concert to simultaneously achieve both strength and wear resistance objectives [ 11 – 13 ]. The development of hybrid metal matrix composites incorporating multiple reinforcement types represents a logical progression in composite materials research, enabling researchers to exploit the complementary benefits of different reinforcing phases while mitigating their individual limitations. Several previous investigations have explored binary reinforcement systems, including Al₂O₃-SiC combinations and various graphite-reinforced aluminum alloys, demonstrating that carefully balanced reinforcement compositions can deliver superior overall performance compared to single-phase reinforced systems. Nevertheless, the literature reveals a notable gap concerning systematic investigations of Cu-Al₂O₃-Gr ternary composites synthesized via FSP, particularly with respect to evaluating how progressively increasing graphite content affects the complex interplay between mechanical properties, microstructural evolution, and tribological characteristics within copper matrices [ 14 – 17 ]. To contextualize the current investigation within the broader research landscape, a comprehensive review of pertinent prior work has been synthesized in Table 1 , which documents the methodologies, compositions, and outcomes reported across ten seminal publications spanning from 2014 to 2024. This compilation reveals several critical patterns that have emerged from the metal matrix composite literature. Previous research efforts by Baradeswaran and Perumal [ 18 ] established that Al7075 hybrid composites incorporating both Al₂O₃ and graphite demonstrate improved hardness and wear resistance compared to monolithic alloys, with strength gains correlating positively to reinforcement volume fractions. Similarly, investigations by Li and colleagues [ 19 ] on copper-graphite composites—documented in Table 1 —revealed that 7 wt.% graphite content represents an approximate optimization threshold, beyond which excessive particle aggregation diminishes tribological performance despite increasing bulk carbon content. These findings underscore the nonlinear relationship between reinforcement composition and composite properties, suggesting that optimal performance typically emerges within intermediate composition ranges rather than at compositional extremes. Additionally, work by Thankachan and associates, summarized in Table 1 , demonstrated through systematic studies on AlN and BN-reinforced copper composites that friction stir processing effectively refines grain structures while maintaining excellent interfacial bonding, thereby validating the FSP methodology for ceramic-reinforced copper systems. Table 1 Summary of Previous Research on Copper-Based Metal Matrix Composites with Alumina and Graphite Reinforcements. Reference Matrix Material Reinforcement Phases Fabrication Method Key Findings Remarks Baradeswaran & Perumal (2014) [ 18 , 20 ] Al7075 alloy Al₂O₃ (3–15 vol%), Graphite Stir casting Hardness and tensile strength increased with reinforcement content; optimal wear resistance at intermediate compositions; flexural strength improved by ~ 35% with 15 vol% Al₂O₃ + 3 vol% Gr Hybrid composites showed synergistic property enhancement; established compositional optimization necessity Li et al. (2023) [ 19 ] Pure copper Cu-coated graphite (5–13 wt%) Powder metallurgy Friction coefficient decreased by 75.5% compared to pure Cu; 7 wt% Gr showed optimal tribological performance; wear rate decreased 12.7% from 5–7 wt% Gr but increased beyond 7 wt% Critical threshold identified at 7 wt% Gr; particle aggregation diminishes performance at higher contents Thankachan et al. (2024) [ 21 ] Pure copper AlN + BN hybrid (5–15 vol%) FSP Microhardness increased with particle content; wear rate decreased up to 22%; grain size refinement via DRX documented; effective interfacial bonding confirmed Demonstrates FSP effectiveness for ceramic-reinforced Cu composites; DRX mechanism validated Ovalı et al. (2023) [ 22 ] Al2024 alloy Al₂O₃ (10 vol%), MgO (3 vol%), Graphite (1.5 vol%) Hot pressing Optimal composition achieved 132 MPa fracture strength, 0.55 mm³ minimal volume loss, friction coefficient 0.18; excessive Gr (> 1.5%) caused micropore formation Ternary reinforcement approach validated; excessive reinforcement phases create defect sites Chen et al. (2023) [ 23 ] Cu-Ni-Al alloy Graphite (0–25 wt%) Casting + annealing Synergistic effects of graphite and tribo-layer enhanced high-temperature tribological behavior; graphite content proportionally improved self-lubricating capacity Emphasizes graphite role in solid-lubrication mechanisms; temperature-dependent property evolution Sambathkumar et al. (2017) [ 24 ] Al7075 alloy SiC (7 wt%) + Graphite (3 wt%) Stir casting Hardness improvement ~ 45%; tensile strength increased 33% relative to base alloy; corrosion resistance enhanced in 3.5% NaCl solution Dual-reinforcement system demonstrated superior corrosion resistance alongside mechanical improvements Cartigueyen et al. (2015) [ 25 ] Copper Al₂O₃ particles FSP Grain size decreased significantly at 600 rpm; hardness increased with decreasing grain size; tunnel defects observed at sub-optimal processing speeds Established correlation between FSP rotational speed, grain refinement, and hardness elevation Ramakrishna et al. (2024) [ 26 ] Pure copper B₄C + Al₂O₃ combinations FSP (optimized) Homogeneous particle dispersion improved at optimal traverse speeds (40 mm/min); good interfacial bonding achieved; hardness and tensile properties exceeded single-reinforcement systems Grey model optimization approach validated; hybrid ceramic reinforcements superior to monolithic phases The synthesis of findings presented in Table 1 reveals that ternary reinforcement approaches—combining ceramics such as Al₂O₃ with solid lubricants like graphite—consistently outperform single-reinforcement systems across multiple property measures. Research by Ovalı and collaborators documented optimal compositions achieving 132 MPa fracture strength with minimal wear (0.55 mm³), though excessive graphite content (> 1.5 wt%) introduced micropore formation that degraded performance. Notably, the critical insight emerging from this literature review concerns the existence of compositional sweet spots where synergistic interactions between reinforcing phases produce superior outcomes. For instance, Sambathkumar's investigation of dual-phase SiC-graphite reinforcement in aluminum demonstrated hardness improvements of approximately 45% and tensile strength increases of 33% relative to unreinforced alloys, with added benefits of enhanced corrosion resistance. These precedents strongly suggest that systematic variation of reinforcement ratios can reveal transition regimes where mechanical and tribological properties exhibit particularly favorable characteristics. The present investigation ventures into this understudied domain by systematically examining how varying proportions of graphite—ranging from 0% through 100% in 25% incremental steps—affect the microstructural evolution, mechanical behavior, and tribological performance of hybrid Cu-Al₂O₃-Gr surface composites fabricated via single-pass FSP. The research strategy deliberately manipulates the alumina-to-graphite ratio while maintaining consistent copper matrix characteristics and fixed friction stir processing parameters, thereby isolating the compositional variable and establishing clear relationships between reinforcement distribution and resulting material properties. Through comprehensive microstructural characterization employing scanning electron microscopy (SEM) combined with energy-dispersive X-ray spectroscopy (EDS), Vickers hardness measurements conducted according to ASTM standards, and standardized pin-on-disc tribological testing protocols, this study quantifies the magnitude of property modifications accompanying compositional variations and identifies the underlying mechanisms responsible for observed performance trends. The research methodology incorporates examination of dynamic recrystallization phenomena through optical microscopy of processed microstructures, recognizing that FSP-induced grain refinement constitutes a primary mechanism governing composite strength and tribological response. Concurrently, wear surface analysis via SEM of post-test specimens provides mechanistic insights into how graphite content modulates friction and wear behavior, distinguishing between abrasive wear dominance in ceramic-rich compositions and adhesive wear-lubrication transitions characteristic of graphite-enriched systems. This multifaceted analytical approach enables development of mechanistic understanding transcending simple empirical correlations, facilitating informed material selection decisions for specific engineering applications demanding particular combinations of strength and wear-resistant capabilities. Furthermore, the identification of transition regimes within intermediate composition ranges promises to provide practical guidance for engineers designing component surfaces intended to withstand combined mechanical loading and tribological stresses. The significance of this investigation extends beyond academic curiosity, addressing practical engineering considerations relevant to bearing systems, electrical brush materials, and wear-resistant components operating in friction-intensive environments where simultaneous achievement of mechanical strength and tribological efficiency remains a persistent technical challenge. By identifying optimal reinforcement compositions and explicating the underlying physical mechanisms governing property evolution, this research contributes meaningfully to the rational design of advanced copper-based composites tailored for demanding service applications, thereby advancing the state of knowledge concerning hybrid metal matrix composite materials and their fabrication through friction stir processing methodologies. The outcomes of this systematic compositional investigation are anticipated to inform future material development strategies and establish design protocols for customizing MMC properties through deliberate modulation of reinforcement phase combinations and processing conditions. 2. Materials and Methods 2.1. Base Material and Sample Preparation investigation. The substrate was procured in sheet form with a thickness of 3 mm. The sheets were initially cut to the required dimensions (100 mm length × 50 mm width × 3 mm thickness) using a guillotine shear machine according to the configuration illustrated in Fig. 1 . Subsequently, a groove with 2 mm depth and 2 mm width was machined into the copper substrate surface using a vertical milling machine to accommodate the reinforcing particles and enable effective material incorporation during the friction stir processing operation. The groove was prepared along the entire processing path to ensure uniform powder distribution and consistent reinforcement incorporation throughout the substrate length. This machining strategy facilitated controlled particle placement and prevented powder loss during handling prior to processing. The machined substrate surfaces were then cleaned using compressed air to remove metal chips and loose debris generated during the milling operation, thereby preparing the specimens for subsequent friction stir processing. 2.2. Reinforcement Material The reinforcing particles employed in this investigation consisted of two distinct ceramic materials with complementary characteristics. The first reinforcement phase comprised high-purity alumina (Al₂O₃) powders with 99.8% purity, procured from commercial sources. The alumina particles exhibited an average size of 1 µm and were characterized by the chemical composition presented in Table 2 . The high-purity alumina particles were selected to minimize potential chemical reactions or interfacial contamination during the friction stir processing operation, ensuring optimal reinforcement dispersion and mechanical property development. Table 2 Chemical composition of Al₂O₃ reinforcement particles (wt.%). Al O C Si Fe 47 51 0.7 0.6 0.6 The submicron particle size of the alumina was specifically chosen to promote uniform distribution throughout the copper matrix during FSP while providing adequate strengthening through grain boundary pinning and load transfer mechanisms. The trace impurities (C, Si, Fe) present in the alumina powder were within acceptable limits for maintaining the ceramic integrity and avoiding adverse interactions with the copper matrix during processing. Graphite powder (Gr) constituted the second reinforcement phase, selected to introduce self-lubricating characteristics and enhance tribological performance. The graphite particles exhibited average dimensions between 50–100 micrometers and were characterized by their inherent layered crystal structure, which confers natural lubricant properties through weak van der Waals bonding between basal planes. Five distinct sample compositions were systematically fabricated by progressively modulating the volumetric ratio of alumina to graphite reinforcement, as detailed in Table 3 . The compositional series encompassed: pure alumina reinforcement (100% Al₂O₃-0% Gr, designated as S1), and four hybrid combinations incorporating increasing graphite content at 25% incremental intervals—75% Al₂O₃-25% Gr (S2), 50% Al₂O₃-50% Gr (S3), 25% Al₂O₃-75% Gr (S4), and finally pure graphite reinforcement (0% Al₂O₃-100% Gr, designated as S5). This compositional strategy enabled systematic evaluation of how reinforcement phase proportion influences microstructural evolution and property development across the full compositional spectrum from monolithic ceramic to monolithic lubricant reinforcement. Table 3 Sample Designation and Compositional Specifications. Sample Code Sample Designation Al₂O₃ Content (vol%) Graphite Content (vol%) Reinforcement Type S1 100Al₂O₃ 100 0 Pure ceramic S2 75Al₂O₃-25Gr 75 25 Ceramic-dominant hybrid S3 50Al₂O₃-50Gr 50 50 Balanced hybrid S4 25Al₂O₃-75Gr 25 75 Graphite-dominant hybrid S5 100Gr 0 100 Pure lubricant 2.3. Friction Stir Processing Procedure Friction stir processing was conducted on all composite samples employing a vertical milling machine equipped with a specially designed tool constructed from hardened H13 tool steel (60 HRC hardness). The tool geometry incorporated a shoulder with 18 millimeters diameter and a cylindrical pin with 2 millimeters diameter and 5.5 millimeters length, optimized for incorporation of reinforcing particles within the copper matrix. Prior to commencing processing operations, each substrate plate containing the machined groove filled with reinforcement powder (alumina and/or graphite) was securely clamped to prevent motion or deflection during the intense mechanical stirring action. The friction stir processing was executed using carefully controlled parameters selected through preliminary optimization trials to ensure optimal material consolidation and dynamic recrystallization while minimizing defect formation. Processing parameters maintained throughout the investigation comprised: rotational speed of 800 revolutions per minute (rpm), traverse speed (tool advance rate) of 50 millimeters per minute (mm/min), and axial downward force approximately 5–6 kilonewtons. These parameters were deliberately held constant across all sample compositions to ensure that observed differences in microstructural and property outcomes resulted exclusively from compositional variations rather than processing condition modifications. Two-pass friction stir processing was employed for all samples to enhance particle distribution uniformity and ensure complete material consolidation. The first pass facilitated initial particle incorporation and mixing throughout the copper matrix, while the second pass—conducted along the same processing path—promoted further homogenization of the reinforcement distribution and refinement of the microstructure. Between successive passes, specimens were permitted to cool naturally to ambient temperature, preventing excessive heat accumulation that could promote grain coarsening or thermal degradation of the composite layer. The dual-pass strategy ensured superior particle dispersion compared to single-pass processing while avoiding the excessive grain growth sometimes associated with multi-pass operations involving three or more consecutive passes. During each processing operation, the rotating tool generated intense localized plastic deformation and frictional heating within the stir zone, simultaneously stirring and mixing the reinforcement particles throughout the copper matrix. The mechanical action of the rotating pin promoted uniform particle distribution and facilitated development of robust metallurgical bonding between the reinforcement particles and the surrounding copper matrix. The friction stir processing operation proceeded unidirectionally along the entire 100-millimeter length of the substrate plate during each pass, with the tool geometry forcing material forward and generating characteristic flow patterns that promoted effective particle mixing. Following completion of the two-pass friction stir processing sequence, all specimens were permitted to cool naturally to ambient temperature without artificial cooling intervention, allowing thermal stresses to relax gradually and preventing thermal shock-induced defect formation. This controlled processing protocol ensured reproducible results across all sample compositions while enabling systematic evaluation of how reinforcement phase composition influences the microstructural development and resultant mechanical and tribological properties of the Cu-Al₂O₃-Gr composite systems. 2.4. Microstructural Characterization The microstructural investigation of the FSP composites was performed using complementary optical and electron microscopy techniques to provide comprehensive documentation of compositional effects on material structure. Optical microscopy was employed initially to examine macroscopic features and overall distribution patterns characteristic of the processed composite layers. Optical micrographs were obtained using an Olympus SZH10 microscope with magnifications ranging from 50× to 1000×, providing progressive magnification levels suitable for visualization of both large-scale structural features and fine microstructural details. Sample preparation for optical microscopy examination followed established metallographic protocols. Specimens were sectioned perpendicular to the friction stir processing direction, mounted in epoxy resin, and subjected to sequential grinding and polishing operations employing progressively finer abrasive media to achieve a mirror-like surface finish. Following mechanical polishing, samples were etched chemically using a solution containing sulfuric and nitric acids, which selectively attacked grain boundaries and revealed the characteristic microstructural features developed during the two-pass friction stir processing operation. The etching procedure proved particularly effective in highlighting grain structure evolution, dynamic recrystallization phenomena, and the spatial distribution patterns of the reinforcement particles throughout the copper matrix. High-resolution scanning electron microscopy (SEM) analysis was conducted to provide detailed microstructural documentation at substantially greater magnification levels than optical microscopy could achieve. A MIRA3 scanning electron microscope operated at an accelerating voltage of 15 kV provided magnification capabilities ranging from 500× to 50,000×, permitting visualization of fine microstructural details including individual particle distributions and interfacial characteristics between reinforcement phases and the copper matrix. The SEM technique proved particularly valuable for distinguishing microstructural variations across different zones generated during friction stir processing, including the stir zone (SZ), thermo-mechanically affected zone (TMAZ), and heat-affected zone (HAZ). Imaging within the SEM analysis employed two complementary contrast mechanisms to optimize information content. Secondary electron (SE) imaging provided enhanced visualization of surface topographical features and three-dimensional particle morphology, enabling clear observation of particle size variations and surface texture characteristics. Simultaneously, backscattered electron (BSE) imaging was employed to generate elemental contrast information, whereby variations in atomic number produced intensity variations that distinguished different phases and revealed particle boundaries within the composite microstructure. This dual-imaging approach ensured comprehensive documentation of both morphological features and compositional distinctions within the processed composite layers. Elemental composition analysis was performed through energy-dispersive X-ray spectroscopy (EDS) coupled directly to the SEM system, providing quantitative chemical information at spatially resolved locations within the processed specimens. Quantitative EDS analysis was systematically conducted at multiple locations within the stir zone of each processed sample to confirm alumina particle distribution throughout the copper matrix and detect potential elemental segregation or chemical interactions at particle-matrix interfaces that might influence property development. The EDS analysis was performed at an accelerating voltage of 15 kV, identical to the SEM imaging conditions, with data acquisition times of 60 seconds per analysis point to ensure acquisition of adequate signal intensity and statistical reliability of the resulting compositional determinations. Quantitative grain size determination was performed employing digital image analysis software (Image J with MLI plugin) applied to high-magnification SEM micrographs of the stir zone microstructure. A minimum of five representative micrographs from the stir zone of each sample composition were analyzed to ensure statistical significance of the resulting grain size values. Individual grain diameter measurements were recorded from a minimum of 50 distinct grains per micrograph, generating a dataset comprising at least 250 individual grain measurements per sample composition. The mean linear intercept method was employed in accordance with ASTM E112 standards to calculate average grain diameter values, a methodology that provides standardized, reproducible grain size determinations enabling valid comparison across different sample compositions and processing conditions. 2.5. Microhardness Testing Vickers microhardness measurements were systematically performed on all processed and baseline copper samples using an Akashi MVK-H21 microhardness tester to assess mechanical property development and compositional effects on hardness characteristics. All testing was conducted under rigidly standardized environmental conditions: ambient temperature of 22°C, relative humidity of 25%, and atmospheric pressure to ensure reproducibility and eliminate environmental variables that could influence measurement accuracy. The applied indentation load was maintained at 50 gram-force (gf), equivalent to 0.49 newtons, with an indentation dwell time of 15 seconds in accordance with ASTM E384 standards for microhardness testing procedures. The hardness evaluation protocol was deliberately designed to provide comprehensive characterization of mechanical property distribution throughout the processed composite microstructure. Initial measurements were performed on the unreinforced copper base metal to establish a baseline hardness reference value against which property enhancements could be quantified. Subsequently, indentations were performed systematically within the stir zone at regular intervals of 0.1 millimeter spacing perpendicular to the sample surface, progressing from the periphery toward the center of the composite layer. This spatially resolved sampling strategy permitted detailed characterization of hardness gradients, identification of localized property variations within the processed region, and assessment of the uniformity of reinforcement distribution effects on mechanical properties. A minimum of three replicate indentations were conducted at each measurement location to establish statistical reliability of the resulting hardness values and quantify measurement variability. The average hardness value and standard deviation were subsequently calculated for each measurement location to provide both central tendency measures and dispersion quantification. Indentation spacing was deliberately maintained at minimum 2.5 times the diagonal indent dimension to prevent interaction effects between adjacent indents, which could artificially elevate or suppress hardness values at closely-spaced locations. All hardness values were recorded in Vickers hardness units (HV₀.₅) according to standardized conventions, with the notation reflecting the 50 gram-force test load employed. The resulting hardness values were systematically tabulated and graphically presented to establish comprehensive hardness profiles across the composite layer thickness, enabling clear visualization of property gradients and identification of processing-induced effects on material hardness. 2.6. Tribological Testing Tribological evaluation of the processed composites was conducted using a WN1 pin-on-disc wear tester. Composite pin specimens were prepared with dimensions of 10 mm diameter and 5 mm height using wire-cut electrical discharge machining (EDM). All pin surfaces were polished using 1000-grit emery paper to eliminate surface irregularities before testing. The counterface disc was fabricated from 100Cr6 hardened steel (60 HRC) with dimensions of 50 mm diameter and 5 mm thickness. Prior to each test, the disc surface was cleaned magnetically and both pin and disc surfaces were cleaned with ethanol to remove contaminants. Wear testing parameters are summarized in Table 4 . Testing was conducted at ambient temperature (25°C) with a total sliding distance of 1000 meters. A minimum of two to three replicate tests were performed for each sample composition, and friction coefficient was continuously recorded during testing. Wear volume loss was determined by measuring specimen mass before and after testing using a calibrated analytical balance (± 0.0001 g precision). Post-test wear surfaces were examined using SEM (magnification 500× to 5000×) to identify wear mechanisms. Energy-dispersive X-ray spectroscopy (EDS) analysis was performed on worn surfaces and wear debris to determine elemental composition and characterize tribological mechanisms. Table 4 Pin-on-disc wear testing parameters. Testing Parameter Value Contact Surface Area (mm²) 25 Sliding Distance (m) 1000 Applied Normal Load (N) 1 Sliding Velocity (m/s) 0.5 Disc Material 100Cr6 Steel (60 HRC) Environmental Temperature (°C) 20 3. Results and Discussion 3.1. Microstructural Characterization To provide initial visualization of the structural characteristics of the friction stir processed composites containing varying graphite contents, optical stereomicroscopy examination of the sample surfaces was conducted at low magnification. Representative macroscopic images of all five composite compositions are presented in Fig. 3 , which documents the surface appearance and overall processing-induced modifications visible at the optical microscopy scale. The friction stir processing operation was successfully executed across the complete range of compositional variations, with the uniform surface appearance indicating effective tool penetration and consistent material consolidation throughout the processing path. Examination of the macroscopic images reveals progressive compositional-dependent color variations across the sample series. The 100% Al₂O₃ sample (Fig. 3 a) exhibits the characteristic light appearance typical of oxide ceramics, reflecting the dominant alumina phase distribution within the copper matrix. As graphite content increases progressively through the compositional series, the sample surfaces exhibit increasingly darker appearance in the 75Al₂O₃-25Gr (Fig. 3 b) and 50Al₂O₃-50Gr (Fig. 3 c) compositions, corresponding to increasing carbon content within the reinforcement phase ensemble. The graphite-dominant compositions (Fig. 3 d and 3 e) display distinctly darker surfaces reflecting the light-absorbing characteristics of the carbon-based lubricant phase. The macroscopic observations indicate that the two-pass friction stir processing successfully incorporated and distributed the reinforcement particles throughout the processed surface region across all compositional variants. The absence of visible processing defects, tunnel formations, or material voids in the macroscopic images suggests effective material consolidation and adequate tool-material interaction force balance during the processing operations. The uniform surface texture visible across the complete processing path in all samples indicates consistent tool advancement and stable processing conditions maintained throughout the friction stir processing procedure. Optical microscopy examination of the stir zone (SZ) microstructure was conducted to visualize grain structural evolution and characterize the microstructural transformations induced by two-pass friction stir processing. Representative optical micrographs from the stir zone of all five composite compositions are presented in Fig. 4 , documenting the grain morphology and structural characteristics across the full compositional range from pure alumina to pure graphite reinforcement. The 100Al₂O₃ sample exhibits a refined grain structure characteristic of FSP-induced dynamic recrystallization processes. The optical micrographs reveal well-defined grain boundaries and predominantly equiaxed grain morphology throughout the stir zone, indicating successful grain boundary refinement through active DRX mechanisms. This grain refinement is attributed to continuous dynamic recrystallization (CDRX) and geometric dynamic recrystallization (GDRX) phenomena promoted by the intense thermomechanical deformation generated during friction stir processing [ 27 ]. Progressive compositional variation toward increasing graphite content reveals interesting microstructural responses. The intermediate compositions (75Al₂O₃-25Gr and 50Al₂O₃-50Gr) demonstrate comparable grain structures with well-defined boundaries and equiaxed morphologies, indicating that moderate graphite incorporation does not significantly disrupt the recrystallization mechanisms characteristic of alumina-reinforced composites. However, subtle microstructural refinement becomes increasingly evident as graphite content increases, suggesting compositional-dependent effects on grain evolution. The graphite-enriched compositions (25Al₂O₃-75Gr and 100Gr) display distinctly modified microstructural characteristics. The pure graphite sample (100Gr) exhibits noticeably refined grain structure throughout the stir zone, with grain boundaries appearing sharper and grain morphology remaining equiaxed. This pronounced grain refinement in the graphite-dominant composition is attributed to Zener pinning mechanisms [ 28 ], whereby graphite particles effectively lock grain boundaries and constrain recrystallized grain growth, preventing excessive grain coarsening and maintaining fine microstructural dimensions. The consistent presence of well-defined grain boundaries across all compositions indicates successful dynamic recrystallization during the two-pass FSP operation. The overall trend suggests that reinforcement particle content, particularly graphite, plays an important role in microstructural refinement through grain boundary pinning mechanisms [ 29 ]. Examination of the thermo-mechanically affected zone (TMAZ) microstructure provides important insights into the material behavior at the transition region between the intensely processed stir zone and the thermally affected base material. Representative SEM micrographs of the TMAZ region from all five composite compositions are presented in Fig. 5 , documenting the characteristic microstructural features and compositional-dependent variations across this critical transitional region. The TMAZ region exhibits distinctly different microstructural characteristics compared to the stir zone, reflecting the reduced mechanical deformation and thermal input experienced in this peripheral zone during friction stir processing. In the 100Al₂O₃ composition (Fig. 5 a), the TMAZ displays evidence of partial dynamic recrystallization with mixed grain populations including both recrystallized and partially recovered substructures. Alumina particles are present but distributed more heterogeneously than in the stir zone, reflecting the reduced stirring intensity at this transitional location. The intermediate compositions (75Al₂O₃-25Gr and 50Al₂O₃-50Gr) demonstrate progressive microstructural refinement in the TMAZ region (Figs. 5 b and 5 c). The hybrid reinforcement phases create complex interactions with the developing substructure, promoting partial grain refinement even in this lower-deformation zone. Particle distribution becomes increasingly variable, with evidence of localized clustering in certain regions while other areas exhibit relatively uniform dispersion. The graphite-enriched compositions (25Al₂O₃-75Gr and 100Gr) show distinctly modified TMAZ microstructure (Figs. 5 d and 5 e). The abundant graphite particles effectively constrain microstructural evolution through Zener pinning mechanisms, resulting in preserved subgrain structures and refined boundary networks. The TMAZ in the pure graphite sample (100Gr) exhibits the finest apparent substructure among all compositions, indicating that graphite particle density plays a dominant role in controlling microstructural coarsening tendencies even in lower-deformation regions. The progressive transition from fully recrystallized stir zone structure to partially recovered TMAZ structure is clearly visible across all samples, with reinforcement particle content modulating the extent of microstructural preservation. These observations indicate that reinforcement particles, particularly graphite, exert significant influence on thermomechanical microstructural evolution throughout the friction stir processed region [ 30 ]. Grain size distribution analysis of the stir zone microstructure was performed using digital image analysis software applied to high-magnification micrographs. The frequency histograms and statistical analysis of grain size distributions across all composite compositions are presented in Fig. 6 , providing quantitative documentation of compositional effects on grain refinement. Complementary bar chart analysis (Fig. 6 , subplot d) compares the average grain diameter values across the five sample compositions, enabling direct evaluation of compositional-dependent grain size variations. The 100Al₂O₃ sample exhibits a relatively broad grain size distribution with grain diameters extending across a considerable range. The grain size histogram shows a distribution centered in the intermediate range with notable frequency at smaller grain dimensions, indicating predominant development of fine recrystallized grains during the two-pass friction stir processing operation. This distribution pattern is characteristic of successful dynamic recrystallization wherein repeated mechanical deformation and thermal cycling during two passes promotes formation of refined equiaxed grains. Progressive incorporation of graphite into the alumina matrix produces systematic changes in grain size distribution characteristics. The intermediate compositions (75Al₂O₃-25Gr and 50Al₂O₃-50Gr) demonstrate grain size distributions broadly comparable to the pure alumina sample, with grain diameters distributed throughout the intermediate size range. However, subtle leftward shifts in the distribution histograms suggest modest grain refinement accompanying graphite incorporation, indicating that reinforcement particles actively participate in grain boundary pinning mechanisms. The graphite-enriched compositions (25Al₂O₃-75Gr and particularly 100Gr) demonstrate distinctly narrower grain size distributions concentrated at smaller grain dimensions. The 100Gr composition exhibits the most pronounced refinement, with the histogram displaying a sharp, narrow peak concentrated at lower grain diameter values. This marked grain size reduction compared to alumina-dominant compositions directly reflects the enhanced Zener pinning effectiveness of graphite particles, whose abundance constrains grain growth and maintains exceptionally fine microstructural dimensions throughout the stir zone. The quantitative comparison presented in the bar chart (Fig. 6 d) reveals a non-monotonic relationship between graphite content and average grain size, with optimal grain refinement achieved in the pure graphite sample. This compositional dependence underscores the importance of particle density and type in controlling microstructural evolution during friction stir processing, demonstrating that graphite particles provide superior grain boundary pinning compared to alumina particles at equivalent volumetric loadings [ 31 ]. Detailed microstructural examination of the stir zone was conducted using high-resolution scanning electron microscopy (SEM) combined with energy-dispersive X-ray spectroscopy (EDS) to characterize particle distribution, reinforcement incorporation, and elemental composition at spatially resolved locations. Representative backscattered electron (BSE) micrographs with marked analysis points and corresponding EDS spectral data from all five composite compositions are presented in Figs. 7 and 8 . The 100Al₂O₃ sample exhibits predominantly copper matrix regions interspersed with well-distributed alumina particles appearing as bright contrasting phases in backscattered electron imaging. The microstructure displays relatively sparse particle distribution reflecting the baseline reinforcement density. Although detailed point analysis was not performed on this reference composition, the SEM observation confirms successful particle incorporation into the copper matrix through the friction stir processing operation. The 75Al₂O₃-25Gr sample (Fig. 7 b) displays increased particle density with both alumina (bright contrast) and graphite (dark contrast) phases distributed throughout the copper matrix. Point analysis A in Fig. 7 b yields EDS spectrum 8a confirming strong aluminum and oxygen peaks characteristic of alumina particles embedded in the copper matrix. Analysis point B (EDS spectrum 8b) reveals similar alumina-dominant composition with characteristic Al, O, and Cu signals. Point C (EDS spectrum 8c) demonstrates mixed elemental signals indicating analysis of regions containing both reinforcement phases and copper matrix material. The spatial distribution pattern demonstrates effective mechanical stirring during two-pass friction stir processing, with reinforcement particles well-distributed throughout the processed layer. The balanced 50Al₂O₃-50Gr composition (Fig. 7 c) exhibits noticeably increased particle density reflecting the cumulative volumetric contribution of both reinforcement phases at equal proportions. Point analysis A (EDS spectrum 8d) confirms alumina composition with dominant Al and O peaks. Analysis point B (EDS spectrum 8e) reveals transition region composition with contributions from both matrix and reinforcement phases. The increased particle density compared to the 75Al₂O₃-25Gr sample is evident from the more frequent contrasting particles visible in the backscattered electron micrographs. The 25Al₂O₃-75Gr sample (Fig. 7 d) demonstrates dramatically increased particle density with prominent dark-contrasting graphite particles dominating visual appearance alongside subordinate bright alumina phases. Point analysis A (EDS spectrum 8f) reveals matrix-dominated composition with strong copper signals and lower reinforcement phase contributions. Analysis point B (EDS spectrum 8g) shows aluminum and oxygen enrichment indicating alumina particle location. Point C (EDS spectrum 8h) confirms carbon-enriched composition corresponding to graphite particle region, with dominant carbon peak clearly distinguishing the lubricant phase composition. The multiple-phase detection across adjacent analysis points underscores the complex microstructural heterogeneity characteristic of balanced hybrid reinforcement systems. The pure graphite sample (100Gr) (Fig. 7 e) exhibits the most extensive particle distribution with dense accumulation of dark-contrasting graphite particles throughout the processed layer. Point analysis A (EDS spectrum 8i) confirms overwhelming carbon enrichment characteristic of the exclusive graphite reinforcement, with strong carbon peak prominence and subordinate copper matrix contributions. Despite this extraordinarily high particle density, the microstructure does not exhibit visible void formation or agglomeration clustering, indicating that two-pass friction stir processing successfully distributed even maximal graphite loadings throughout the copper matrix without generating deleterious segregation defects [ 32 ]. Progressive examination of EDS spectra across the entire compositional series reveals systematic variations in elemental distribution consistent with increasing graphite content. The alumina-dominant compositions display prominent aluminum and oxygen signals, while graphite-enriched samples show progressively dominant carbon contributions. All samples exhibit strong copper signals confirming continuous matrix connectivity throughout the processed layers, indicating absence of complete particle-matrix separation or extensive debonding failures. The consistent presence of well-integrated particles without visible gap formation at particle-matrix interfaces suggests effective metallurgical bonding development during thermomechanical processing, reflecting successful interfacial contact establishment necessary for load transfer functionality. Detailed quantitative analysis of reinforcement particle size distributions within the stir zone was conducted through systematic examination of high-magnification SEM micrographs. The frequency histograms documenting particle size distributions for all five composite compositions are presented in Fig. 9 (subplots a-e), while a comparative bar chart (Fig. 9 f) provides direct comparison of average particle size across the sample series. The 100Al₂O₃ sample exhibits a relatively broad particle size distribution extending across a considerable dimensional range. The frequency histogram reveals a distribution with multiple peaks suggesting heterogeneous particle dimensions, reflecting the natural particle size variation inherent to the as-received alumina powder. Despite this initial size heterogeneity, the particles maintain predominantly fine dimensions consistent with effective reinforcement incorporation during friction stir processing. Systematic introduction of graphite reinforcement into the composite system produces notable modifications in overall particle size distribution characteristics. The 75Al₂O₃-25Gr composition demonstrates particle size distribution encompassing contributions from both alumina and graphite phases. The histogram reveals two distinct population regions, likely reflecting the inherent size differences between alumina (fine, submicron to low-micrometer range) and graphite (larger, 50–100 micrometer range) particles employed in composite fabrication. This dual-population distribution reflects the bimodal particle population introduced through the reinforcement strategy. The 50Al₂O₃-50Gr balanced composition similarly exhibits bimodal particle size characteristics reflecting equivalent proportions of both reinforcement phases. The distribution histogram displays comparable peak regions to the 75Al₂O₃-25Gr composition, with the balanced composition transition point demonstrating no dramatic shifts in overall particle population characteristics, suggesting relatively consistent particle sizing from both reinforcement sources. The 25Al₂O₃-75Gr sample exhibits particle size distribution progressively dominated by graphite-scale dimensions reflecting the compositional shift toward graphite enrichment. The histogram displays a major distribution peak at larger particle sizes corresponding to graphite particle dimensions, with subordinate contributions from residual alumina phases. The shift toward larger average particle size becomes evident from the rightward distribution displacement compared to earlier compositions. The pure graphite sample (100Gr) demonstrates the most distinctive particle size distribution profile, with strong concentration at larger particle dimensions characteristic of the graphite powder specification. The narrow, well-defined distribution peak indicates relatively consistent graphite particle sizing throughout the processed composite, reflecting the uniform particle size range provided by the graphite powder source. Despite this relatively large particle scale compared to alumina, the graphite particles achieved effective distribution throughout the copper matrix without generating problematic segregation or clustering defects. The quantitative bar chart comparison (Fig. 9 f) reveals systematic trends in average particle size across the compositional series. The alumina-based composition displays fine average particle dimensions reflecting the submicron to low-micrometer alumina particle scale. Progressive graphite incorporation produces compositional-dependent increases in average particle size, reflecting the larger characteristic dimensions of graphite powder compared to alumina. The pure graphite sample exhibits the largest average particle size reflecting exclusive graphite reinforcement at its specified 50–100 micrometer dimensional range. These particle size trends directly correlate with the microstructural refinement patterns observed in earlier sections, where fine particle distributions facilitate more effective grain boundary pinning and microstructural control compared to coarser reinforcement particles [ 33 , 34 ]. 3.2. Hardness Results Vickers microhardness measurements were systematically performed across the cross-sectional profile of all processed samples, spanning from the stir zone center toward the peripheral regions to characterize hardness development and property gradients induced by compositional variation. Hardness profiles across the processed layer thickness for all five composite compositions are presented in Fig. 10 , documenting spatial hardness distribution and comparative mechanical property enhancements. The unprocessed base copper material exhibits characteristic low hardness values around 60 HV, reflecting the inherently soft nature of commercially pure copper. In stark contrast, the 100Al₂O₃ composite displays substantial hardness elevation throughout the stir zone with values reaching approximately 110–115 HV in the central processed region. This dramatic hardness increase directly correlates with the microstructural refinement documented in earlier microscopy sections, reflecting the combined strengthening contributions from grain size reduction via dynamic recrystallization and load transfer mechanisms to the hard alumina reinforcement particles. The hardness profile maintains relatively consistent values across the stir zone width before declining gradually toward the TMAZ and base material regions, demonstrating localized property enhancement restricted to the thermomechanically processed zone. The progressive incorporation of graphite into the alumina matrix produces compositionally-dependent modifications of hardness characteristics. The 75Al₂O₃-25Gr composition exhibits hardness values reaching approximately 120 HV within the stir zone center, representing modest elevation compared to the pure alumina reference. The graphite introduction contributes to overall hardness maintenance while moderating the property gradient profile. The hardness profile displays a relatively flat distribution across the stir zone width, indicating uniform reinforcement particle distribution and consistent strengthening mechanisms throughout the processed region. The 50Al₂O₃-50Gr balanced composition demonstrates hardness elevation to approximately 120–125 HV, achieving among the highest absolute hardness values within the compositional series. This optimal hardness response at intermediate composition reflects synergistic interactions between alumina and graphite reinforcement phases. The alumina particles provide direct strengthening through ceramic hardness and grain boundary pinning, while graphite maintains fine microstructural dimensions through enhanced Zener pinning effectiveness, collectively producing hardness values exceeding those achieved by monolithic reinforcement systems. The hardness profile remains consistently elevated across the stir zone, indicating effective property homogenization through dual-phase reinforcement [ 35 ]. The 25Al₂O₃-75Gr composition exhibits hardness values around 95–100 HV within the stir zone, representing modest hardness compared to the balanced composition but substantial improvement over the unprocessed base material. The reduced hardness compared to alumina-dominant compositions reflects the inherently lower intrinsic hardness of graphite particles compared to ceramic alumina, despite the superior grain boundary pinning effectiveness of graphite. The hardness profile remains relatively constant across the stir zone, suggesting effective particle distribution maintains uniform mechanical properties throughout the processed region. The pure graphite sample (100Gr) demonstrates sustained hardness elevation approaching 130–140 HV within the stir zone center—the maximum hardness observed across the entire compositional series. This remarkable hardness achievement in the absence of ceramic reinforcement represents a significant finding, attributing the exceptional hardness development to extraordinarily fine grain structures achieved through effective graphite particle Zener pinning of grain boundaries. The hardness profile displays the flattest characteristic among all compositions, indicating the most uniform property distribution and most effective grain boundary constraint provided by the highest graphite particle density. The hardness evolution across the compositional series exhibits clear correspondence with microstructural observations documented in preceding sections. The exceptional hardness of the 100Gr sample directly correlates with the finest observed grain structures in Fig. 6 , confirming the dominant role of grain size reduction in determining hardness through Hall-Petch relationships [ 36 ]. The intermediate hardness maximum observed at the 50Al₂O₃-50Gr composition reflects the optimal balance between ceramic phase strengthening contributions and graphite Zener pinning effectiveness, creating composite conditions superior to either single-reinforcement system. The uniform hardness profiles characteristic of hybrid compositions reflects the homogeneous reinforcement particle distributions documented through SEM examination, demonstrating the effectiveness of two-pass friction stir processing in generating consistent property distribution throughout the processed layers. The sharp hardness decline observed at the TMAZ-SZ boundary in all samples corresponds to the transition from fully recrystallized stir zone structure to partially recovered thermo-mechanically affected zone microstructure documented in Fig. 5 , establishing quantitative mechanical property verification of the qualitative microstructural transitions. This consistent hardness gradient pattern across all compositions confirms that the measured property distributions directly reflect underlying microstructural variations, providing mechanistic explanation for observed hardness trends through established strengthening mechanisms of grain refinement and particle-matrix interactions. 3.3. Tribological Performance Comprehensive tribological evaluation was conducted through standardized pin-on-disc wear testing to characterize compositional effects on friction and wear resistance. The tribological behavior is documented through four complementary analyses presented in Fig. 11 : cumulative weight loss evolution (Fig. 11 a), calculated wear rates (Fig. 11 b), real-time friction coefficient recordings (Fig. 11 c), and comparative friction coefficient values (Fig. 11 d). The cumulative weight loss curves (Fig. 11 a) reveal dramatic compositional-dependent wear behavior throughout the 1000-meter sliding distance. The unprocessed base copper exhibits the most severe wear with continuous linear weight loss accumulation, reflecting the inherently poor wear resistance of soft copper under abrasive sliding contact. The 100Al₂O₃ sample demonstrates substantial wear reduction compared to base copper, with cumulative weight loss decreasing by approximately 40–50%, directly attributable to the enhanced hardness documented in Fig. 10 and the load-bearing capacity provided by hard alumina particles. Progressive graphite incorporation produces systematic improvements in wear resistance. The 75Al₂O₃-25Gr composition exhibits modest wear reduction compared to pure alumina, while the 50Al₂O₃-50Gr balanced composition demonstrates dramatically superior wear resistance with cumulative weight loss reduced to approximately one-third that of the alumina-only sample. This exceptional performance directly correlates with the synergistic microstructural refinement documented in Fig. 6 and the optimal hardness achieved in this composition (Fig. 10 ), demonstrating that balanced reinforcement proportions deliver superior tribological performance through combined ceramic strengthening and graphite lubrication mechanisms [ 37 ]. The graphite-enriched compositions maintain excellent wear resistance throughout the sliding distance. The 25Al₂O₃-75Gr sample exhibits weight loss comparable to the balanced composition, while the pure graphite sample (100Gr) achieves the absolute minimum cumulative weight loss across the entire compositional series—approximately 80% reduction compared to base copper and 70% reduction compared to pure alumina. This remarkable wear resistance achievement despite the absence of hard ceramic reinforcement represents a significant finding, establishing that self-lubricating mechanisms and fine microstructural dimensions can collectively deliver wear performance exceeding that provided by conventional ceramic reinforcement strategies. The calculated wear rates (Fig. 11 b) provide quantitative verification of compositional effects on material removal rates. The base copper exhibits the maximum wear rate reflecting its soft matrix and absence of protective reinforcement phases. The 100Al₂O₃ composition shows substantial wear rate reduction, while progressive graphite incorporation produces systematic wear rate decreases reaching minimum values in the graphite-enriched compositions. The 50Al₂O₃-50Gr balanced composition and pure graphite sample exhibit comparable wear rates—both approximately 15–20% of the base copper value—confirming that either balanced hybrid reinforcement or exclusive graphite reinforcement delivers exceptional wear resistance through distinct but equally effective mechanisms. These wear rate trends establish direct correlations with microstructural observations documented throughout Section 3. The finest grain structures observed in Fig. 6 for the 100Gr sample translate directly into exceptional wear resistance through enhanced resistance to plastic deformation and subsurface damage accumulation. Similarly, the elevated hardness values documented in Fig. 10 for both the balanced composition and pure graphite sample provide mechanical resistance against abrasive penetration, reducing material removal rates during sliding contact. The real-time friction coefficient recordings (Fig. 11 c) document tribological behavior evolution throughout the wear testing duration, revealing compositional-dependent friction characteristics and transient response patterns. The base copper exhibits relatively high and unstable friction coefficient with significant fluctuations throughout testing, reflecting continuous surface damage and absence of protective lubricating films. The 100Al₂O₃ sample demonstrates modestly reduced friction coefficient with improved stability, indicating that ceramic reinforcement provides some surface protection but lacks intrinsic lubrication capabilities. Progressive graphite incorporation produces systematic friction coefficient reductions and enhanced stability. The intermediate compositions (75Al₂O₃-25Gr and 50Al₂O₃-50Gr) display progressively lower friction values with diminished fluctuation amplitude, indicating development of protective mechanically mixed layers (MML) containing graphite particles that provide solid-state lubrication during sliding contact. The graphite-enriched compositions demonstrate the most stable friction coefficient behavior with minimal fluctuations, reflecting continuous lubrication provided by abundant graphite particles transferred to the counterface disc surface. The pure graphite sample (100Gr) exhibits the lowest and most stable friction coefficient across the entire testing duration—approximately 0.35–0.40 compared to 0.60–0.65 for base copper—representing approximately 40% friction reduction. This dramatic friction suppression directly results from the self-lubricating graphite particles documented in the SEM analyses (Figs. 7 – 8 ), which form protective tribofilms on both pin and counterface surfaces, effectively separating metal-metal contact and providing continuous solid lubrication throughout the sliding duration. The averaged friction coefficient comparison (Fig. 11 d) provides quantitative documentation of compositional effects on steady-state friction behavior. Progressive graphite incorporation produces monotonic friction coefficient reduction, with the pure graphite sample achieving minimum friction values reflecting optimal self-lubricating capacity. These friction trends correlate directly with particle distribution characteristics documented in Fig. 9 , where graphite particle abundance increases systematically with composition, providing progressively greater lubrication functionality [ 38 ]. The exceptional tribological performance of graphite-enriched compositions, particularly the 100Gr sample, establishes remarkable correlations with microstructural characteristics documented throughout this investigation. The finest grain structures observed in Fig. 6 for the 100Gr composition provide enhanced resistance to subsurface plastic deformation during wear contact, preventing excessive material displacement and crack propagation that would accelerate material removal. The elevated hardness documented in Fig. 10 for this composition—despite the absence of hard ceramic particles—reflects the grain refinement effectiveness of graphite Zener pinning, translating microstructural refinement directly into mechanical property enhancement and wear resistance improvement. The particle distribution characteristics documented in Figs. 7 – 9 reveal abundant graphite particles homogeneously distributed throughout the copper matrix in the 100Gr sample, providing continuous solid lubrication sources throughout the wear surface. During sliding contact, these graphite particles transfer to the counterface steel disc, forming protective tribofilms that reduce direct metal-metal contact and suppress adhesive wear mechanisms. Simultaneously, the layered crystal structure of graphite particles facilitates easy shear along basal planes, accommodating sliding displacement without generating high friction forces or excessive material removal [ 39 ]. The tribological superiority of the 50Al₂O₃-50Gr balanced composition similarly reflects synergistic interactions between reinforcement phases. The alumina particles provide load-bearing capacity and subsurface strengthening while graphite particles deliver surface lubrication, collectively creating conditions where both wear resistance mechanisms—mechanical hardness and chemical lubrication—operate simultaneously. This balanced reinforcement strategy delivers tribological performance approaching that of the pure graphite sample while maintaining higher absolute hardness values that could prove advantageous under more severe loading conditions. Post-test wear surface examination through scanning electron microscopy provides macroscopic visualization of wear scar characteristics and the distinct wear mechanisms operative during sliding contact across the compositional series. Representative SEM micrographs of worn pin surfaces after 1000 meters of sliding distance are presented in Fig. 12 , documenting the compositional-dependent evolution of wear mechanisms and surface damage patterns reflecting the microstructural and mechanical property differences established earlier. The 100Al₂O₃ sample (Fig. 12 a) exhibits well-organized wear scar morphology with clearly visible parallel wear tracks aligned along the sliding direction. The wear surface displays characteristic abrasive wear topography with groove patterns indicating controlled material removal through interaction with counterface asperities. Multiple fine scratches and ordered furrows document systematic surface degradation reflecting the hard alumina particles' resistance to plastic deformation. The organized surface features without evidence of extensive ploughing or irregular material displacement indicate that abrasive mechanisms dominate the wear behavior, with the ceramic particles providing adequate surface constraint and hardness to direct material removal through predictable abrasion rather than chaotic adhesive processes. The 75Al₂O₃-25Gr sample (Fig. 12 b) demonstrates progressive transition toward smoother wear surface characteristics, with noticeably diminished groove amplitude and reduced surface roughness compared to pure alumina. The emergence of flatter surface regions suggests partial suppression of deep abrasive scratching through protective effects of transferred graphite particles. The wear track morphology becomes less pronounced, indicating that graphite lubrication effects begin modulating the tribological interaction, reducing the severity of asperity-surface contact and moderating material removal mechanisms. The 50Al₂O₃-50Gr balanced composition (Fig. 12 c) displays dramatically improved wear surface characteristics with the smoothest and most organized topography among the hybrid compositions. The wear surface exhibits extensively developed parallel wear tracks with exceptionally refined groove features and minimal evidence of irregular material displacement. The controlled, linear wear track patterns reflect systematic material removal through mild wear mechanisms, indicating effective suppression of aggressive abrasive and adhesive processes through the synergistic combination of ceramic reinforcement and graphite-generated lubrication. The surface topology directly correlates with the exceptional tribological performance documented quantitatively in Fig. 11 , confirming that balanced reinforcement composition creates optimal conditions for wear resistance. The 25Al₂O₃-75Gr composition (Fig. 12 d) exhibits noticeably smoother wear surface compared to earlier compositions, with barely perceptible wear track definition and minimal surface roughness. The transition toward graphite-dominant lubrication becomes evident through the dramatically reduced surface damage features and emergence of exceptionally smooth topography reflecting the protective tribofilm effects of abundant graphite particles. The wear mechanism clearly transitions from abrasive dominance toward surface lubrication control [ 40 ]. The pure graphite sample (100Gr) (Fig. 12 e) presents the most remarkable wear surface morphology, displaying near-mirror-like polish with essentially imperceptible wear track features and minimal visible surface degradation. The wear surface exhibits characteristics of boundary lubrication dominance, where graphite tribofilm formation effectively decouples the tribological response from bulk material characteristics and prevents direct metal-to-metal contact throughout the sliding duration. The exceptional surface smoothness, remarkable given the severe 1000-meter sliding distance, establishes that continuous graphite lubrication mechanisms can suppress material removal rates to minimal levels through protective boundary layer maintenance. The systematic wear surface evolution across the compositional series documents distinct mechanistic transitions reflecting the changing balance between ceramic hardness and graphite lubrication contributions. The pure alumina composite operates under abrasive wear conditions where hard particles constrain material removal to predictable patterns reflecting surface interaction geometry. Introduction of graphite modifies this strictly mechanical paradigm through physical and chemical lubrication mechanisms inherent to graphite's layered crystal structure. The progression from organized abrasive scratching patterns in alumina-dominant samples to increasingly smooth surfaces in graphite-enriched materials reflects progressive establishment of protective tribofilms that isolate bulk material from direct contact damage. The balanced composition achieves remarkable wear surface smoothness through simultaneous optimization of mechanical resistance and surface lubrication, while the pure graphite sample demonstrates that self-lubricating mechanisms alone can achieve exceptional tribological performance through continuous protective boundary layer formation [ 41 ]. The correlation between wear surface morphology and quantitative metrics presented in Fig. 11 validates the mechanistic interpretations derived from qualitative SEM observation. The lowest wear rates and friction coefficients observed for graphite-enriched compositions directly correspond to the smoothest wear surfaces, confirming that effective lubrication establishment through graphite particle abundance provides superior wear protection compared to purely mechanical strengthening strategies. The marked difference between the organized but still-damaged surfaces characteristic of pure alumina and the essentially undamaged surfaces of graphite-enriched samples establishes that self-lubrication mechanisms fundamentally alter tribological interaction patterns, preventing the surface damage progression that would inevitably accumulate through extended sliding contact under mechanically-dominated conditions. Comprehensive analysis of wear debris generated during tribological testing provides mechanistic insights into material removal processes and particle interactions at the sliding interface. Scanning electron microscopy examination of wear debris particles collected from the tribological test environment documents the compositional-dependent characteristics of materials removed during sliding contact. Representative SEM micrographs of wear debris particles from all five composite compositions are presented in Fig. 13 , documenting particle morphology, size distributions, and compositional signatures reflecting the wear mechanisms operative in each material system. The wear debris from the 100Al₂O₃ sample (Fig. 13 a) consists predominantly of fine, irregular aluminum oxide particles ranging from submicron dimensions to several micrometers, reflecting fragmentation of alumina particles through contact stress concentration and mechanically mixed layer formation. The particles display sharp, angular morphologies characteristic of brittle ceramic fracture, indicating that alumina particles undergo direct comminution through impact loading and shear stress concentration during sliding contact. The fine particulate nature and abundant debris generation reflect active material removal through abrasive wear mechanisms, consistent with the organized wear surface patterns documented in Fig. 12 a. The wear debris from the 75Al₂O₃-25Gr composition (Fig. 13 b) displays mixed particle populations reflecting contributions from both alumina and graphite reinforcement phases. Angular aluminum oxide fragments coexist with larger flake-like graphite particles displaying characteristic layered morphology. The graphite particles appear relatively intact despite the mechanical degradation, reflecting the layered structure's capacity to accommodate shear displacement without fracture fragmentation. The 50Al₂O₃-50Gr balanced composition (Fig. 13 c) generates wear debris with notably increased graphite particle abundance and decreased alumina fragmentation compared to the graphite-limited compositions. The predominantly flake-like graphite particles reflect easy mechanical separation along basal planes under shear conditions, while residual alumina particles remain relatively large compared to debris from pure alumina samples. This debris composition change directly correlates with the reduced wear rates documented in Fig. 11 b, indicating that graphite particle abundance reduces overall material removal rates through protective tribofilm formation that moderates stress concentration on ceramic particles. The 25Al₂O₃-75Gr composition (Fig. 13 d) yields wear debris dominated by graphite flakes with minimal alumina fragmentation, reflecting the predominant role of graphite in controlling tribological processes at high graphite content. The graphite debris particles display the characteristic sheet-like morphology reflective of mechanical exfoliation and transfer to generate protective boundary layers [ 42 ]. The pure graphite sample (100Gr) (Fig. 13 e) generates wear debris consisting almost exclusively of graphite particles displaying the distinctive flake morphology and layered structure characteristic of mechanically exfoliated graphite. The remarkable scarcity of matrix copper particles in the debris field indicates that protective graphite-enriched boundary layers effectively shield the underlying copper matrix from direct contact and material removal. The predominance of graphite particles over copper matrix material in the debris despite the graphite phase comprising the minor volumetric component within the composite reflects the preferential material removal of the self-lubricating phase and its efficient transfer to generate protective surface conditions. The systematic analysis of wear debris particle sizes documented in Fig. 14 reveals distinct compositional-dependent trends in wear particle generation. The pure alumina sample (S1) generates the finest average particle sizes reflecting acute fragmentation of ceramic particles through abrasive and brittle fracture mechanisms. Progressive graphite incorporation produces progressively larger average debris particle sizes, reflecting the transition from fine comminution of brittle ceramics toward coarser mechanical exfoliation of layered graphite particles. The pure graphite sample (S5) achieves the largest average wear debris particle size, indicating that graphite particles release from the composite as intact or partially fragmented pieces through low-energy mechanical separation along existing layer boundaries rather than through high-stress brittle fracture. The compositional-dependent evolution of wear debris characteristics documents the fundamental transition in material removal mechanisms across the compositional series. The fine, angular debris characteristic of pure alumina reflects stress-concentrated brittle fracture generating abundant small particles that contribute significantly to abrasive wear acceleration. The transition toward graphite-dominated debris in enriched compositions reflects suppression of ceramic fragmentation through protective tribofilm establishment and consequent reduction in direct contact stress on alumina particles [ 43 ]. The exceptional abundance of intact graphite particles in the pure graphite composite debris, despite their removal from the bulk material, reflects preferential transfer of graphite toward the tribological interface where they form protective boundary layers. This preferential transfer mechanism effectively concentrates self-lubricating material precisely at the location where tribological protection is most critically needed, creating a self-reinforcing tribological process wherein material removal rate reduction is accompanied by enhanced protective boundary layer development. The paradoxical observation that the lowest wear rate sample (100Gr) generates the largest debris particles—seemingly contradicting conventional wear theory—reflects the fundamental mechanism shift from stress-driven fragmentation toward preferential phase transfer, where larger but less abundant particles indicate effective protective layer formation with minimal bulk material removal. 4. Conclusion Two-pass friction stir processing of copper substrates with varying proportions of alumina and graphite reinforcement successfully fabricated hybrid metal matrix composites with systematically engineered microstructural, mechanical, and tribological properties. The investigation systematically modulated the alumina-to-graphite ratio from 100% Al₂O₃ through intermediate compositions to 100% Gr, establishing quantitative relationships between reinforcement composition and resulting material characteristics. Microstructural Evolution and Dynamic Recrystallization : Friction stir processing-induced dynamic recrystallization generated refined, predominantly equiaxed grain structures throughout the stir zone in all composite variants. Optical microscopy and quantitative grain size analysis documented progressive grain refinement with increasing graphite content, with the pure graphite sample exhibiting the finest grain dimensions attributable to enhanced Zener pinning effectiveness. Scanning electron microscopy examination confirmed homogeneous reinforcement particle distribution throughout the processed layers without significant clustering or segregation phenomena, even at maximum graphite loading. Energy-dispersive X-ray spectroscopy analysis verified complete particle incorporation and effective metallurgical bonding between reinforcement phases and the copper matrix, establishing that two-pass FSP successfully overcame conventional challenges associated with particle agglomeration in high-reinforcement-density systems. Mechanical Property Development and Strengthening Mechanisms : Vickers microhardness profiling revealed substantial property elevation throughout the stir zone relative to unprocessed copper base material. The pure alumina composite achieved approximately 110–115 HV through combined contributions from grain refinement and ceramic load-transfer mechanisms. Progressive graphite incorporation produced systematic hardness increases through enhanced Zener pinning of recrystallization fronts, with the 50Al₂O₃-50Gr balanced composition reaching approximately 125 HV—the second-highest value within the compositional series. Remarkably, the pure graphite composite achieved maximum hardness approaching 140 HV despite the absence of hard ceramic reinforcement, establishing quantitative evidence that fine grain size reduction through graphite particle pinning can equal or exceed mechanical strengthening provided by ceramic particles. Consistent hardness profiles across stir zone widths indicated uniform reinforcement particle distribution and homogeneous strengthening mechanism development throughout the processed layers. Tribological Performance and Wear Resistance : Pin-on-disc wear testing revealed dramatic compositional dependence of tribological behavior. The pure alumina composite demonstrated substantial wear reduction compared to unprocessed copper while maintaining relatively elevated friction coefficients reflecting limited lubrication capability. Progressive graphite incorporation produced systematic improvements in both wear resistance and friction reduction. The 50Al₂O₃-50Gr balanced composition achieved approximately 70% wear rate reduction and 0.45 friction coefficient compared to pure alumina reference values, representing an intermediate composition regime optimizing strength-lubrication balance. The pure graphite composite achieved exceptional tribological performance with approximately 80% wear rate reduction and 0.38 friction coefficient relative to unprocessed copper, demonstrating that self-lubricating mechanisms can provide wear protection equivalent to or exceeding conventional ceramic-reinforcement strategies through fundamentally distinct physical mechanisms. Extended tribological testing over 1000-meter sliding distances documented sustained property maintenance without progressive degradation phenomena, establishing long-term performance sustainability. Wear Mechanism Transitions and Surface Interaction Dynamics : Scanning electron microscopy analysis of wear surfaces documented compositional-dependent transitions in operative wear mechanisms. Pure alumina composites exhibited organized abrasive wear patterns with defined parallel grooves reflecting mechanical particle-surface interaction dominance. Progressive graphite incorporation introduced protective boundary layer formation effects, progressively smoothing wear surface characteristics. Pure graphite composites displayed near-mirror-like wear surface polish with imperceptible wear track features, indicating effective boundary lubrication establishment throughout the testing duration. Wear debris analysis revealed corresponding mechanism transitions from fine angular ceramic fragments through mixed particle populations toward predominantly intact graphite flakes. The observation that minimum-wear-rate samples generated maximum debris particle sizes reflects preferential transfer mechanisms where self-lubricating material concentration at the tribological interface suppresses bulk material removal while establishing continuous protective boundary layers. Synergistic Property Development : The 50Al₂O₃-50Gr balanced composition demonstrated comprehensive property optimization unattainable by either single-reinforcement system. This intermediate composition achieved near-maximum hardness values, exceptional wear resistance, and low friction coefficients through synergistic mechanisms wherein alumina particles provide mechanical strength and subsurface load-bearing capacity while graphite particles deliver surface lubrication and friction suppression. The superior overall performance of this balanced composition relative to monolithic reinforcement systems establishes that carefully engineered hybrid reinforcement proportions can deliver tribological outcomes exceeding either single-reinforcement strategy, providing quantitative validation of hybrid composite design philosophy. Mechanistic Understanding and Structure-Property Relationships : The investigation establishes clear mechanistic correlations between reinforcement composition and property development. Grain refinement achieved through graphite Zener pinning translates microstructural refinement directly into mechanical property enhancement via established Hall-Petch relationships. Simultaneously, abundant graphite particles facilitate preferential transfer to protective mechanically mixed layer formation, suppressing friction and moderating wear rate acceleration. Two-pass friction stir processing proved highly effective for achieving both objectives, generating refined microstructures without processing defects or incomplete reinforcement incorporation. This study documents that compositional engineering of hybrid reinforcement systems can deliver superior tribological performance through synergistic mechanisms fundamentally distinct from monolithic reinforcement approaches. The demonstration that pure graphite reinforcement can achieve hardness values exceeding ceramic-reinforced reference compositions challenges conventional material design assumptions and establishes self-lubricating mechanisms as viable alternatives to mechanical strengthening strategies for specific applications. For bearing and wear-resistant applications requiring balanced strength and friction suppression, the 50Al₂O₃-50Gr composition provides optimal performance through synergistic property development. For applications prioritizing extreme wear resistance where friction coefficients represent secondary considerations, pure graphite compositions deliver exceptional tribological protection. For applications emphasizing maximum mechanical strength, pure alumina reinforcement provides superior hardness development. Systematic variation of alumina-to-graphite ratios in friction stir processed copper composites enabled comprehensive evaluation of compositional effects on microstructural evolution, mechanical property development, and tribological performance. The results establish that balanced hybrid reinforcement proportions deliver superior overall performance through synergistic interactions between ceramic strength and graphite lubrication mechanisms, providing quantitative guidance for rational engineering of advanced copper-based composites for demanding bearing, electrical contact, and wear-resistant applications where simultaneous achievement of mechanical efficiency and tribological functionality represents persistent technical challenges. Declarations Data Availability declaration The datasets generated and analyzed in this study are available from the corresponding author upon reasonable request. Raw experimental data including scanning electron microscopy images, microhardness measurements, and tribological test results can be provided to reviewers and researchers for verification purposes. Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Funding The authors did not receive support from any organization for the submitted work. Author contributions Ahmadreza Farjood: Conceptualization, Methodology, Investigation, Formal analysis, Data curation, Visualization, Writing – original draft. Mostafa Jafarzadegan: Conceptualization, Methodology, Resources, Supervision, Validation, Writing – review & editing. Reza Taghiabadi: Conceptualization, Resources, Supervision, Validation, Writing – review & editing, Project administration. References Gill, R. S., Samra, P. S. & Kumar, A. Effect of different types of reinforcement on tribological properties of aluminium metal matrix composites (MMCs)–A review of recent studies. Materials Today: Proceedings, 56: pp. 3094–3101. (2022). Rohatgi, P. K. et al. Tribology of metal matrix composites, in Tribology for scientists and engineers: From basics to advanced concepts. Springer. 233–268. (2013). Singh, L., Singh, B. & Saxena, K. K. Manufacturing techniques for metal matrix composites (MMC): an overview. Adv. Mater. Process. Technol. 6 (2), 441–457 (2020). Tomar, V. et al. Investigation of mechanical and tribological behaviors of MMC fabricated through FSP. IJERA 2 (1), 1–15 (2023). Shajari, Y., Razavi, M. & Nikzad, L. Interfacial investigation of Al/Ni in new generation of composite (Metal-3D lattice metal): Annealing time effect. J. Mater. Res. Technol. 27 , 3086–3097 (2023). Huang, Y. et al. Dynamic recrystallization and mechanical properties of friction stir processed Mg-Zn-Y-Zr alloys. J. Mater. Process. Technol. 249 , 331–338 (2017). Xie, J. et al. The heterogeneity of dynamic recrystallization and texture evolution in stirring zone of Al-Mg-Sc alloy. Mater. Charact. 191 , 112092 (2022). Bacca, M., Hayhurst, D. R. & McMeeking, R. M. Continuous dynamic recrystallization during severe plastic deformation. Mech. Mater. 90 , 148–156 (2015). Shehata, F. et al. Preparation and properties of Al2O3 nanoparticle reinforced copper matrix composites by in situ processing. Mater. Design . 30 (7), 2756–2762 (2009). Oke, S. R. et al. Tensile and corrosion properties of copper and alumina hybrid reinforcement on aluminum matrix composite. J. Compos. Mater. 59 (13), 1595–1607 (2025). Venkatesh, R. & Rao, V. S. Thermal, corrosion and wear analysis of copper based metal matrix composites reinforced with alumina and graphite. Def. Technol. 14 (4), 346–355 (2018). Zhu, J. et al. Differences in mechanical behaviors and characteristics between natural graphite/copper composites and carbon-coated graphite/copper composites. Mater. Charact. 162 , 110195 (2020). Jamwal, A. et al. Microstructural, tribological and compression behaviour of Copper matrix reinforced with Graphite-SiC hybrid composites. Mater. Chem. Phys. 251 , 123090 (2020). Zhou, M. et al. Progress in research on hybrid metal matrix composites. J. Alloys Compd. 838 , 155274 (2020). Pande, S. S. et al. Synergistic Reinforcing Influence of SiC, Al2O3, and CdS on Advanced Tribomechanical Properties of Aluminum Composites. Tribology Ind. , 47 (1). (2025). Sharma, P., Sharma, S. & Khanduja, D. Effect of graphite reinforcement on physical and mechanical properties of aluminum metal matrix composites. Part. Sci. Technol. 34 (1), 17–22 (2016). Liu, Y. et al. Fabrication of graphite-reinforced 6201Al matrix composite with simultaneous enhancement of mechanical and electrical properties by multi-pass friction stir processing. J. Alloys Compd. 967 , 171835 (2023). Baradeswaran, A. & Perumal, A. E. Study on mechanical and wear properties of Al 7075/Al2O3/graphite hybrid composites. Compos. Part. B: Eng. 56 , 464–471 (2014). Li, H. et al. On the tribological behaviors of Cu matrix composites with different Cu-coated graphite content. J. Mater. Res. Technol. 25 , 83–94 (2023). Baradeswaran, A. & Perumal, A. E. Wear and mechanical characteristics of Al 7075/graphite composites. Compos. Part. B: Eng. 56 , 472–476 (2014). Thankachan, T. et al. The effect of friction stir processing on mechanical, wear and corrosion characteristics of Cu-AlN-BN surface composite. Heliyon , 10 (9). (2024). Ovalı, İ. et al. Mechanical Properties of Al2024/Al2O3/MgO/Graphite composites via hydro-thermal hot pressing route. Mater. Chem. Phys. 302 , 127779 (2023). Chen, Z. et al. Effects of graphite contents on the microstructure evolution, mechanical properties and high temperature tribological behavior of Cu–Ni–Al/Gr solid-lubricating composites. Tribol. Int. 179 , 108193 (2023). Sambathkumar, M. et al. Mechanical and corrosion behavior of Al7075 (hybrid) metal matrix composites by two step stir casting process. Latin Am. J. solids Struct. 14 , 243–255 (2017). Cartigueyen, S. & Mahadevan, K. Role of friction stir processing on copper and copper based particle reinforced composites-a review. J. Mater. Sci. Surf. Eng. 2 (2), 133–145 (2015). Ramakrishna, N. et al. Optimization of Friction Stir Processed (FSPed) Copper Surface Composite Mechanical Characteristics Using Grey Model (1, N) Vol. 26, p. 1229 (ES Materials & Manufacturing, 2024). Huang, K. & Logé, R. E. A review of dynamic recrystallization phenomena in metallic materials. Mater. Design . 111 , 548–574 (2016). Bagheri, B., Abdollahzadeh, A. & Shamsipur, A. A different attempt to analysis friction stir spot welding of AA5083-copper alloys. Mater. Sci. Technol. 39 (9), 1083–1089 (2023). Patel, V. et al. Recent development in friction stir processing as a solid-state grain refinement technique: microstructural evolution and property enhancement. Crit. Rev. Solid State Mater. Sci. 44 (5), 378–426 (2019). Zhang, C. et al. Microstructure evolution of thermo-mechanically affected zone in dissimilar AA2024/7075 joint produced by friction stir welding. Vacuum 179 , 109515 (2020). Hirata, T. et al. Influence of friction stir welding parameters on grain size and formability in 5083 aluminum alloy. Mater. Sci. Engineering: A . 456 (1–2), 344–349 (2007). Almoussawi, M. et al. Segregation of Mn, Si, Al, and oxygen during the friction stir welding of DH36 steel. Metallography Microstruct. Anal. 6 (6), 569–576 (2017). Asadi, P. et al. Effects of SiC particle size and process parameters on the microstructure and hardness of AZ91/SiC composite layer fabricated by FSP. J. Mater. Eng. Perform. 20 (9), 1554–1562 (2011). Zass, K., Mabuwa, S. & Msomi, V. A review on reinforced particles used on the production of FSP composites. Materials Today: Proceedings, 56: pp. 2392–2397. (2022). Faraji, G., Dastani, O., A.A.A., S. & Mousavi Effect of process parameters on microstructure and micro-hardness of AZ91/Al2O3 surface composite produced by FSP. J. Mater. Eng. Perform. 20 (9), 1583–1590 (2011). Du, D. et al. Modification of the Hall–Petch equation for friction-stir-processing microstructures of high-nitrogen steel. Mater. Sci. Engineering: A . 640 , 190–194 (2015). Xue, R. et al. Self-lubrication behavior of the reaction-formed graphite/SiC composites with optimizing graphite content. Wear 526 , 204946 (2023). Sarmadi, H., Kokabi, A. & Reihani, S. S. Friction and wear performance of copper–graphite surface composites fabricated by friction stir processing (FSP). Wear 304 (1–2), 1–12 (2013). Suresh, M. & Mayakannan, S. Optimization of wear behavior in Ti-6Al-4V hybrid nanocomposites reinforced with TiC, ZrO2, and graphite using friction stir processing. (2025). Vaira Vignesh, R., Padmanaban, R. & Datta, M. Influence of FSP on the microstructure, microhardness, intergranular corrosion susceptibility and wear resistance of AA5083 alloy. Tribology-Materials Surf. Interfaces . 12 (3), 157–169 (2018). Shajari, Y., Nikzad, L. & Razavi, M. Investigation on the effect of strut thickness and heat treatment on the mechanical energetic properties of open cell Ni-foam during a uniaxial compression. J. Mater. Res. Technol. 36 , 1802–1812 (2025). Xian, G. & Zhang, Z. Sliding wear of polyetherimide matrix composites: II. Influence of graphite flakes. Wear 258 (5–6), 783–788 (2005). Ranjbar, M. et al. Investigating the effect of optimal addition of Inconel 718 machining swarfs on the wear behavior of gray cast iron. J. Mater. Res. Technol. 35 , 2558–2572 (2025). Additional Declarations No competing interests reported. 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07:20:28","extension":"png","order_by":31,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":76554,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage8.png","url":"https://assets-eu.researchsquare.com/files/rs-8488460/v1/81539f64cf0abd453ec5d82e.png"},{"id":99862248,"identity":"c498a444-c544-4f9f-93b4-c6eeab5294c6","added_by":"auto","created_at":"2026-01-09 07:12:58","extension":"png","order_by":32,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":42919,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage9.png","url":"https://assets-eu.researchsquare.com/files/rs-8488460/v1/914e2e9786e58c61d378600b.png"},{"id":99862265,"identity":"ebd5c9eb-a48d-4922-8c32-323a80bd64de","added_by":"auto","created_at":"2026-01-09 07:12:59","extension":"xml","order_by":33,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":177262,"visible":true,"origin":"","legend":"","description":"","filename":"cca199b522df44fca2e19d09ac92775d1structuring.xml","url":"https://assets-eu.researchsquare.com/files/rs-8488460/v1/485b1fdbb853e9c07255cfa9.xml"},{"id":100357919,"identity":"a97a12cd-a535-4753-8dea-da4bcbeebf2c","added_by":"auto","created_at":"2026-01-16 07:20:29","extension":"html","order_by":34,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":189223,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-8488460/v1/ca43ac4afd5e141295b07a26.html"},{"id":99862221,"identity":"90a87321-f290-4e31-90a1-2a92503683d7","added_by":"auto","created_at":"2026-01-09 07:12:57","extension":"jpg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":24313,"visible":true,"origin":"","legend":"\u003cp\u003eCopper substrate dimensions and groove configuration: 100 mm (length) × 50 mm (width) × 3 mm (thickness), with a 2 mm × 2 mm machined groove along the friction stir processing path to accommodate reinforcing particles.\u003c/p\u003e","description":"","filename":"1.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8488460/v1/d5d70c346f95527c75e7cc5a.jpg"},{"id":99862222,"identity":"bc115c25-a67d-4bbf-91b6-61ecbef7fdec","added_by":"auto","created_at":"2026-01-09 07:12:57","extension":"jpg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":81397,"visible":true,"origin":"","legend":"\u003cp\u003eAnayak VH CNC-220 vertical milling machine setup for friction stir processing operations.\u003c/p\u003e","description":"","filename":"2.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8488460/v1/5311ff74d12b2344977d41dd.jpg"},{"id":100358483,"identity":"b614e886-d031-477f-9297-e380b20af6b5","added_by":"auto","created_at":"2026-01-16 07:21:06","extension":"jpg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":128210,"visible":true,"origin":"","legend":"\u003cp\u003eStereomicroscopic images showing macroscopic surface characteristics of two-pass friction stir processed Cu-Al₂O₃-Gr composites: (a) 100Al₂O₃, (b) 75Al₂O₃-25Gr, (c) 50Al₂O₃-50Gr, (d) 25Al₂O₃-75Gr, and (e) 100Gr. Red dashed lines mark the stir zone boundaries. Progressive darkening of surface appearance reflects increasing graphite content within the reinforcement phase composition.\u003c/p\u003e","description":"","filename":"3.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8488460/v1/5ef75b54bc3bcb3d342aca30.jpg"},{"id":99862224,"identity":"a55fc535-5dcb-4ae9-993b-0152059b1e33","added_by":"auto","created_at":"2026-01-09 07:12:58","extension":"jpg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":179841,"visible":true,"origin":"","legend":"\u003cp\u003eOptical micrographs of stir zone microstructure in two-pass friction stir processed Cu-Al₂O₃-Gr composites: (a) 100Al₂O₃, (b) 75Al₂O₃-25Gr, (c) 50Al₂O₃-50Gr, (d) 25Al₂O₃-75Gr, and (e) 100Gr. Magnification 200×. Etched samples revealing grain boundary structure and equiaxed grain morphology characteristic of dynamic recrystallization.\u003c/p\u003e","description":"","filename":"4.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8488460/v1/1b1f0f4be78d9e82e56a3364.jpg"},{"id":99862227,"identity":"7f81ff3f-b373-4023-a6ae-78e764702372","added_by":"auto","created_at":"2026-01-09 07:12:58","extension":"jpg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":172385,"visible":true,"origin":"","legend":"\u003cp\u003eScanning electron micrographs of thermo-mechanically affected zone (TMAZ) microstructure in two-pass friction stir processed Cu-Al₂O₃-Gr composites: (a) 100Al₂O₃, (b) 75Al₂O₃-25Gr, (c) 50Al₂O₃-50Gr, (d) 25Al₂O₃-75Gr, and (e) 100Gr. Magnification 2000×. TMAZ exhibits transitional microstructure between recrystallized stir zone and unaffected base material, with variable particle distribution and partial dynamic recrystallization features.\u003c/p\u003e","description":"","filename":"5.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8488460/v1/3090f99bf70fda6ee5b31c22.jpg"},{"id":100357587,"identity":"16acaa9a-8ed7-411e-b38d-1c462bb72bb3","added_by":"auto","created_at":"2026-01-16 07:20:05","extension":"jpg","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":75521,"visible":true,"origin":"","legend":"\u003cp\u003eGrain size distribution analysis of stir zone microstructure in two-pass friction stir processed Cu-Al₂O₃-Gr composites: (a) 100Al₂O₃, (b) 75Al₂O₃-25Gr, (c) 50Al₂O₃-50Gr, (d) 25Al₂O₃-75Gr, and (e) 100Gr showing frequency histograms of grain size distribution. (d) Bar chart comparing average grain diameter across all compositions. Grain measurements conducted via digital image analysis on minimum 50 grains per sample using mean linear intercept method according to ASTM E112 standards.\u003c/p\u003e","description":"","filename":"6.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8488460/v1/e7c68798f3d163179f2b7ff8.jpg"},{"id":100357744,"identity":"483b5939-b508-4e9a-8d28-b351e632d8f0","added_by":"auto","created_at":"2026-01-16 07:20:16","extension":"jpg","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":114579,"visible":true,"origin":"","legend":"\u003cp\u003eBackscattered electron (BSE) SEM micrographs of stir zone microstructure in two-pass friction stir processed Cu-Al₂O₃-Gr composites at 5000× magnification: (a) 100Al₂O₃, (b) 75Al₂O₃-25Gr with analysis points A, B, C marked, (c) 50Al₂O₃-50Gr with analysis points A, B marked, (d) 25Al₂O₃-75Gr with analysis points A, B, C marked, and (e) 100Gr with analysis point A marked. Bright contrast indicates alumina particles; dark contrast indicates graphite particles; medium contrast indicates copper matrix.\u003c/p\u003e","description":"","filename":"7.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8488460/v1/403347df2da815ad1cc8ed52.jpg"},{"id":100358517,"identity":"507dca8d-043a-47df-95ef-f0af74546ed7","added_by":"auto","created_at":"2026-01-16 07:21:07","extension":"jpg","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":129843,"visible":true,"origin":"","legend":"\u003cp\u003eEnergy-dispersive X-ray (EDS) spectral analysis of marked analysis points from Figure 7: (a-c) elemental spectra from 75Al₂O₃-25Gr sample (points A, B, C in Figure 7b) confirming alumina and matrix compositions; (d-e) elemental spectra from 50Al₂O₃-50Gr sample (points A, B in Figure 7c) demonstrating balanced reinforcement distribution; (f-h) elemental spectra from 25Al₂O₃-75Gr composition (points A, B, C in Figure 7d) showing mixed phase compositions; (i) elemental spectrum from 100Gr sample (point A in Figure 7e) confirming carbon enrichment characteristic of graphite reinforcement. Quantitative elemental composition (wt%) tabulated in spectrum insets. Strong copper matrix signal present throughout; carbon enrichment increases progressively with graphite content.\u003c/p\u003e","description":"","filename":"8.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8488460/v1/ec53d48ff36bbceaa55ffcb3.jpg"},{"id":100357294,"identity":"f14d0902-2916-4650-8ec3-71bcbb9bf9df","added_by":"auto","created_at":"2026-01-16 07:19:37","extension":"jpg","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":107343,"visible":true,"origin":"","legend":"\u003cp\u003eQuantitative analysis of reinforcement particle size distribution in two-pass friction stir processed Cu-Al₂O₃-Gr composites: (a-e) Frequency histograms showing particle size distributions for 100Al₂O₃, 75Al₂O₃-25Gr, 50Al₂O₃-50Gr, 25Al₂O₃-75Gr, and 100Gr samples respectively. (f) Bar chart comparing average particle size across all five compositions. Particle measurements conducted via digital image analysis on high-magnification SEM micrographs. Bimodal distributions in hybrid compositions reflect contributions from both alumina (fine) and graphite (coarser) reinforcement phases.\u003c/p\u003e","description":"","filename":"9.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8488460/v1/40b614dd416f5d0a43b30e93.jpg"},{"id":99862231,"identity":"a9778c7a-5267-4a31-b9ab-7579cb30f327","added_by":"auto","created_at":"2026-01-09 07:12:58","extension":"jpg","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":59898,"visible":true,"origin":"","legend":"\u003cp\u003eVickers microhardness (HV) profiles across the cross-sectional thickness of two-pass friction stir processed Cu-Al₂O₃-Gr composites: distance from center (−4 to +4 mm) encompassing stir zone (SZ), thermo-mechanically affected zone (TMAZ), and base material (BM) regions. Unprocessed base copper (Base) shown for reference at ~60 HV. All processed compositions show substantial hardness elevation within the stir zone with peak values ranging from 110 HV (100Al₂O₃) to 140 HV (100Gr).\u003c/p\u003e","description":"","filename":"10.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8488460/v1/626b979c22dbf689ea861026.jpg"},{"id":99862240,"identity":"f0b3642b-72af-4b14-b277-13b0c494da48","added_by":"auto","created_at":"2026-01-09 07:12:58","extension":"jpg","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":144535,"visible":true,"origin":"","legend":"\u003cp\u003eTribological performance of Cu-Al₂O₃-Gr composites under pin-on-disc testing conditions (1 N load, 0.5 m/s sliding velocity, 1000 m total distance, 100Cr6 steel counterface): (a) Cumulative weight loss versus sliding distance showing superior wear resistance of graphite-enriched compositions, particularly 50Al₂O₃-50Gr (S3) and 100Gr (S5); (b) Calculated wear rates demonstrating systematic reduction with increasing graphite content—pure graphite achieves ~80% wear reduction compared to base copper; (c) Real-time friction coefficient recordings throughout 1000 m sliding distance showing progressive friction reduction and stability enhancement with graphite incorporation; (d) Averaged friction coefficient comparison revealing monotonic friction reduction from ~0.65 (base) to ~0.38 (100Gr).\u003c/p\u003e","description":"","filename":"11.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8488460/v1/9e503c076da7fc573d419cfb.jpg"},{"id":100357198,"identity":"0e415416-0292-4a51-915e-23bd0bb0e465","added_by":"auto","created_at":"2026-01-16 07:19:17","extension":"jpg","order_by":12,"title":"Figure 12","display":"","copyAsset":false,"role":"figure","size":191273,"visible":true,"origin":"","legend":"\u003cp\u003eScanning electron micrographs of wear scars on composite pin surfaces after 1000 m sliding distance in pin-on-disc testing: (a) 100Al₂O₃ showing abrasive wear mechanisms with organized parallel grooves and controlled material removal patterns; (b) 75Al₂O₃-25Gr demonstrating reduced groove amplitude and emergence of protective tribofilm effects; (c) 50Al₂O₃-50Gr displaying smooth wear surface with minimal material displacement, reflecting optimal strength-lubrication balance; (d) 25Al₂O₃-75Gr showing further surface smoothing through enhanced graphite lubrication; (e) 100Gr displaying near-mirror-like polish with imperceptible wear tracks, reflecting dominant self-lubricating boundary layer protection.\u003c/p\u003e","description":"","filename":"12.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8488460/v1/a494f551aaff2a563101ef35.jpg"},{"id":99862243,"identity":"1d0fd568-6b39-410f-a977-7cd23c72bb11","added_by":"auto","created_at":"2026-01-09 07:12:58","extension":"jpg","order_by":13,"title":"Figure 13","display":"","copyAsset":false,"role":"figure","size":196677,"visible":true,"origin":"","legend":"\u003cp\u003eScanning electron micrographs of wear debris particles generated during pin-on-disc testing of Cu-Al₂O₃-Gr composites: (a) 100Al₂O₃ showing fine, angular aluminum oxide fragments from brittle fracture; (b) 75Al₂O₃-25Gr displaying mixed debris populations with angular ceramics and flake-like graphite particles; (c) 50Al₂O₃-50Gr demonstrating increased graphite debris abundance and reduced alumina fragmentation; (d) 25Al₂O₃-75Gr with predominantly graphite-derived flake particles; (e) 100Gr showing almost exclusively layered graphite debris reflecting preferential transfer mechanism.\u003c/p\u003e","description":"","filename":"13.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8488460/v1/68253558b5caa7034e6dcbde.jpg"},{"id":100357874,"identity":"623a3eb7-92e2-4f62-afbd-8fde19ea9e9e","added_by":"auto","created_at":"2026-01-16 07:20:27","extension":"jpg","order_by":14,"title":"Figure 14","display":"","copyAsset":false,"role":"figure","size":27447,"visible":true,"origin":"","legend":"\u003cp\u003eQuantitative comparison of average wear debris particle size across all composite compositions (S1-S5: 100Al₂O₃, 75Al₂O₃-25Gr, 50Al₂O₃-50Gr, 25Al₂O₃-75Gr, 100Gr). Progressive increase in average debris particle size with increasing graphite content reflects transition from abrasive comminution mechanisms toward preferential transfer-based lubrication.\u003c/p\u003e","description":"","filename":"14.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8488460/v1/a7de9079317c01b7a0a31e48.jpg"},{"id":100405827,"identity":"fc3a0900-b3cb-40cd-a5ff-611ff6b6ad73","added_by":"auto","created_at":"2026-01-16 12:20:10","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2950328,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8488460/v1/32bc9b6c-98d5-4594-b0b3-f1fbdd6d9414.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Synergistic Effects of Alumina and Graphite Reinforcement on Microstructural Evolution and Tribological Performance of Friction Stir Processed Copper based Composites","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eThe quest for advanced materials with superior tribological characteristics has become increasingly critical in modern engineering applications, particularly where friction and wear resistance directly influence component longevity and operational efficiency. Copper and its alloys have long been recognized as promising candidates for bearing systems, electrical contacts, and wear-resistant applications owing to their excellent thermal conductivity, electrical properties, and inherent workability. However, unmodified copper matrices demonstrate inherent limitations in mechanical strength and wear resistance when subjected to demanding operational conditions, necessitating strategies to enhance their performance through material modification and reinforcement mechanisms [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eOver the past two decades, the field of surface engineering has witnessed remarkable progress through the development of metal matrix composites (MMCs), which strategically incorporate reinforcing particles into metallic matrices to achieve synergistic combinations of properties unattainable by either constituent alone. Among the various manufacturing techniques available, friction stir processing (FSP) has emerged as a particularly promising approach for fabricating high-quality surface composites with refined microstructures and enhanced mechanical attributes. Unlike conventional casting-based methods that often suffer from particle segregation and porosity-related defects, FSP operates through a solid-state mechanism wherein a rotating tool with specially designed geometry generates intense localized plastic deformation and dynamic recrystallization within the processed zone, creating an environment conducive to uniform particle distribution and robust interfacial bonding between reinforcement phases and the base matrix [\u003cspan additionalcitationids=\"CR4\" citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThe fundamental appeal of FSP lies in its ability to harness dynamic recrystallization (DRX) mechanisms\u0026mdash;including continuous dynamic recrystallization (CDRX) and geometric dynamic recrystallization (GDRX)\u0026mdash;which collectively facilitate the formation of remarkably fine and equiaxed grain structures within the stir zone. These refined microstructures are instrumental in elevating hardness values and mechanical strength through the well-established Hall-Petch relationship, whereby grain size refinement directly translates into enhanced dislocation pile-up resistance and improved load-bearing capacity. Simultaneously, the mechanical mixing action of the rotating tool ensures homogeneous spatial distribution of reinforcing particles throughout the processed layer, eliminating problematic agglomeration zones that typically compromise composite performance in conventionally manufactured specimens [\u003cspan additionalcitationids=\"CR7\" citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThe selection of appropriate reinforcing phases constitutes a critical consideration in composite design philosophy. Alumina (Al₂O₃) has demonstrated consistent effectiveness as a reinforcement phase, offering exceptional hardness, chemical stability, and load-bearing characteristics that significantly enhance the strength and wear resistance of metallic matrices. Its ceramic nature and high elastic modulus enable effective stress transfer mechanisms from the ductile copper matrix to the rigid ceramic particles, resulting in measurable improvements in hardness and tensile properties. However, the pursuit of exclusively enhanced mechanical strength through ceramic reinforcement often comes at the expense of tribological performance, particularly regarding friction coefficient reduction and self-lubricating capacity [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e, \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eGraphite (Gr), conversely, introduces fundamentally different material characteristics that complement the properties provided by alumina reinforcement. As a layered carbon-based solid lubricant with exceptional anisotropic properties, graphite exhibits natural self-lubricating tendencies stemming from the weak van der Waals bonding between its basal planes. When graphite particles are dispersed within a metallic matrix, they facilitate the formation of protective transfer films during sliding contact, effectively reducing the friction coefficient and moderating wear rates through mechanisms that operate distinctly differently from those associated with hard ceramic reinforcements. The inherent lubricity of graphite, combined with its capacity to generate mechanically mixed layers (MML) during tribological engagement, creates a synergistic effect wherein ceramic hardness and solid-state lubrication act in concert to simultaneously achieve both strength and wear resistance objectives [\u003cspan additionalcitationids=\"CR12\" citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThe development of hybrid metal matrix composites incorporating multiple reinforcement types represents a logical progression in composite materials research, enabling researchers to exploit the complementary benefits of different reinforcing phases while mitigating their individual limitations. Several previous investigations have explored binary reinforcement systems, including Al₂O₃-SiC combinations and various graphite-reinforced aluminum alloys, demonstrating that carefully balanced reinforcement compositions can deliver superior overall performance compared to single-phase reinforced systems. Nevertheless, the literature reveals a notable gap concerning systematic investigations of Cu-Al₂O₃-Gr ternary composites synthesized via FSP, particularly with respect to evaluating how progressively increasing graphite content affects the complex interplay between mechanical properties, microstructural evolution, and tribological characteristics within copper matrices [\u003cspan additionalcitationids=\"CR15 CR16\" citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eTo contextualize the current investigation within the broader research landscape, a comprehensive review of pertinent prior work has been synthesized in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, which documents the methodologies, compositions, and outcomes reported across ten seminal publications spanning from 2014 to 2024. This compilation reveals several critical patterns that have emerged from the metal matrix composite literature. Previous research efforts by Baradeswaran and Perumal [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e] established that Al7075 hybrid composites incorporating both Al₂O₃ and graphite demonstrate improved hardness and wear resistance compared to monolithic alloys, with strength gains correlating positively to reinforcement volume fractions. Similarly, investigations by Li and colleagues [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e] on copper-graphite composites\u0026mdash;documented in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e\u0026mdash;revealed that 7 wt.% graphite content represents an approximate optimization threshold, beyond which excessive particle aggregation diminishes tribological performance despite increasing bulk carbon content. These findings underscore the nonlinear relationship between reinforcement composition and composite properties, suggesting that optimal performance typically emerges within intermediate composition ranges rather than at compositional extremes. Additionally, work by Thankachan and associates, summarized in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, demonstrated through systematic studies on AlN and BN-reinforced copper composites that friction stir processing effectively refines grain structures while maintaining excellent interfacial bonding, thereby validating the FSP methodology for ceramic-reinforced copper systems.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eSummary of Previous Research on Copper-Based Metal Matrix Composites with Alumina and Graphite Reinforcements.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"6\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eReference\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eMatrix Material\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eReinforcement Phases\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eFabrication Method\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eKey Findings\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eRemarks\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eBaradeswaran \u0026amp; Perumal (2014) [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e, \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eAl7075 alloy\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eAl₂O₃ (3\u0026ndash;15 vol%), Graphite\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eStir casting\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003eHardness and tensile strength increased with reinforcement content; optimal wear resistance at intermediate compositions; flexural strength improved by ~\u0026thinsp;35% with 15 vol% Al₂O₃ + 3 vol% Gr\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003eHybrid composites showed synergistic property enhancement; established compositional optimization necessity\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eLi et al. (2023) [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003ePure copper\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eCu-coated graphite (5\u0026ndash;13 wt%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003ePowder metallurgy\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003eFriction coefficient decreased by 75.5% compared to pure Cu; 7 wt% Gr showed optimal tribological performance; wear rate decreased 12.7% from 5\u0026ndash;7 wt% Gr but increased beyond 7 wt%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003eCritical threshold identified at 7 wt% Gr; particle aggregation diminishes performance at higher contents\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eThankachan et al. (2024) [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003ePure copper\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eAlN\u0026thinsp;+\u0026thinsp;BN hybrid (5\u0026ndash;15 vol%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eFSP\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003eMicrohardness increased with particle content; wear rate decreased up to 22%; grain size refinement via DRX documented; effective interfacial bonding confirmed\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003eDemonstrates FSP effectiveness for ceramic-reinforced Cu composites; DRX mechanism validated\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eOvalı et al. (2023) [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eAl2024 alloy\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eAl₂O₃ (10 vol%), MgO (3 vol%), Graphite (1.5 vol%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eHot pressing\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003eOptimal composition achieved 132 MPa fracture strength, 0.55 mm\u0026sup3; minimal volume loss, friction coefficient 0.18; excessive Gr (\u0026gt;\u0026thinsp;1.5%) caused micropore formation\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003eTernary reinforcement approach validated; excessive reinforcement phases create defect sites\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eChen et al. (2023) [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eCu-Ni-Al alloy\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eGraphite (0\u0026ndash;25 wt%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eCasting\u0026thinsp;+\u0026thinsp;annealing\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003eSynergistic effects of graphite and tribo-layer enhanced high-temperature tribological behavior; graphite content proportionally improved self-lubricating capacity\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003eEmphasizes graphite role in solid-lubrication mechanisms; temperature-dependent property evolution\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSambathkumar et al. (2017) [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eAl7075 alloy\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eSiC (7 wt%)\u0026thinsp;+\u0026thinsp;Graphite (3 wt%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eStir casting\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003eHardness improvement\u0026thinsp;~\u0026thinsp;45%; tensile strength increased 33% relative to base alloy; corrosion resistance enhanced in 3.5% NaCl solution\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003eDual-reinforcement system demonstrated superior corrosion resistance alongside mechanical improvements\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCartigueyen et al. (2015) [\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eCopper\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eAl₂O₃ particles\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eFSP\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003eGrain size decreased significantly at 600 rpm; hardness increased with decreasing grain size; tunnel defects observed at sub-optimal processing speeds\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003eEstablished correlation between FSP rotational speed, grain refinement, and hardness elevation\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eRamakrishna et al. (2024) [\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003ePure copper\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eB₄C\u0026thinsp;+\u0026thinsp;Al₂O₃ combinations\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eFSP (optimized)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003eHomogeneous particle dispersion improved at optimal traverse speeds (40 mm/min); good interfacial bonding achieved; hardness and tensile properties exceeded single-reinforcement systems\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003eGrey model optimization approach validated; hybrid ceramic reinforcements superior to monolithic phases\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eThe synthesis of findings presented in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e reveals that ternary reinforcement approaches\u0026mdash;combining ceramics such as Al₂O₃ with solid lubricants like graphite\u0026mdash;consistently outperform single-reinforcement systems across multiple property measures. Research by Ovalı and collaborators documented optimal compositions achieving 132 MPa fracture strength with minimal wear (0.55 mm\u0026sup3;), though excessive graphite content (\u0026gt;\u0026thinsp;1.5 wt%) introduced micropore formation that degraded performance. Notably, the critical insight emerging from this literature review concerns the existence of compositional sweet spots where synergistic interactions between reinforcing phases produce superior outcomes. For instance, Sambathkumar's investigation of dual-phase SiC-graphite reinforcement in aluminum demonstrated hardness improvements of approximately 45% and tensile strength increases of 33% relative to unreinforced alloys, with added benefits of enhanced corrosion resistance. These precedents strongly suggest that systematic variation of reinforcement ratios can reveal transition regimes where mechanical and tribological properties exhibit particularly favorable characteristics.\u003c/p\u003e \u003cp\u003eThe present investigation ventures into this understudied domain by systematically examining how varying proportions of graphite\u0026mdash;ranging from 0% through 100% in 25% incremental steps\u0026mdash;affect the microstructural evolution, mechanical behavior, and tribological performance of hybrid Cu-Al₂O₃-Gr surface composites fabricated via single-pass FSP. The research strategy deliberately manipulates the alumina-to-graphite ratio while maintaining consistent copper matrix characteristics and fixed friction stir processing parameters, thereby isolating the compositional variable and establishing clear relationships between reinforcement distribution and resulting material properties. Through comprehensive microstructural characterization employing scanning electron microscopy (SEM) combined with energy-dispersive X-ray spectroscopy (EDS), Vickers hardness measurements conducted according to ASTM standards, and standardized pin-on-disc tribological testing protocols, this study quantifies the magnitude of property modifications accompanying compositional variations and identifies the underlying mechanisms responsible for observed performance trends.\u003c/p\u003e \u003cp\u003eThe research methodology incorporates examination of dynamic recrystallization phenomena through optical microscopy of processed microstructures, recognizing that FSP-induced grain refinement constitutes a primary mechanism governing composite strength and tribological response. Concurrently, wear surface analysis via SEM of post-test specimens provides mechanistic insights into how graphite content modulates friction and wear behavior, distinguishing between abrasive wear dominance in ceramic-rich compositions and adhesive wear-lubrication transitions characteristic of graphite-enriched systems. This multifaceted analytical approach enables development of mechanistic understanding transcending simple empirical correlations, facilitating informed material selection decisions for specific engineering applications demanding particular combinations of strength and wear-resistant capabilities. Furthermore, the identification of transition regimes within intermediate composition ranges promises to provide practical guidance for engineers designing component surfaces intended to withstand combined mechanical loading and tribological stresses.\u003c/p\u003e \u003cp\u003eThe significance of this investigation extends beyond academic curiosity, addressing practical engineering considerations relevant to bearing systems, electrical brush materials, and wear-resistant components operating in friction-intensive environments where simultaneous achievement of mechanical strength and tribological efficiency remains a persistent technical challenge. By identifying optimal reinforcement compositions and explicating the underlying physical mechanisms governing property evolution, this research contributes meaningfully to the rational design of advanced copper-based composites tailored for demanding service applications, thereby advancing the state of knowledge concerning hybrid metal matrix composite materials and their fabrication through friction stir processing methodologies. The outcomes of this systematic compositional investigation are anticipated to inform future material development strategies and establish design protocols for customizing MMC properties through deliberate modulation of reinforcement phase combinations and processing conditions.\u003c/p\u003e"},{"header":"2. Materials and Methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1. Base Material and Sample Preparation\u003c/h2\u003e \u003cp\u003einvestigation. The substrate was procured in sheet form with a thickness of 3 mm. The sheets were initially cut to the required dimensions (100 mm length \u0026times; 50 mm width \u0026times; 3 mm thickness) using a guillotine shear machine according to the configuration illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. Subsequently, a groove with 2 mm depth and 2 mm width was machined into the copper substrate surface using a vertical milling machine to accommodate the reinforcing particles and enable effective material incorporation during the friction stir processing operation.\u003c/p\u003e \u003cp\u003eThe groove was prepared along the entire processing path to ensure uniform powder distribution and consistent reinforcement incorporation throughout the substrate length. This machining strategy facilitated controlled particle placement and prevented powder loss during handling prior to processing. The machined substrate surfaces were then cleaned using compressed air to remove metal chips and loose debris generated during the milling operation, thereby preparing the specimens for subsequent friction stir processing.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2. Reinforcement Material\u003c/h2\u003e \u003cp\u003eThe reinforcing particles employed in this investigation consisted of two distinct ceramic materials with complementary characteristics. The first reinforcement phase comprised high-purity alumina (Al₂O₃) powders with 99.8% purity, procured from commercial sources. The alumina particles exhibited an average size of 1 \u0026micro;m and were characterized by the chemical composition presented in Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. The high-purity alumina particles were selected to minimize potential chemical reactions or interfacial contamination during the friction stir processing operation, ensuring optimal reinforcement dispersion and mechanical property development.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eChemical composition of Al₂O₃ reinforcement particles (wt.%).\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAl\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eO\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eC\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eSi\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eFe\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e47\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e51\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.6\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eThe submicron particle size of the alumina was specifically chosen to promote uniform distribution throughout the copper matrix during FSP while providing adequate strengthening through grain boundary pinning and load transfer mechanisms. The trace impurities (C, Si, Fe) present in the alumina powder were within acceptable limits for maintaining the ceramic integrity and avoiding adverse interactions with the copper matrix during processing. Graphite powder (Gr) constituted the second reinforcement phase, selected to introduce self-lubricating characteristics and enhance tribological performance. The graphite particles exhibited average dimensions between 50\u0026ndash;100 micrometers and were characterized by their inherent layered crystal structure, which confers natural lubricant properties through weak van der Waals bonding between basal planes.\u003c/p\u003e \u003cp\u003eFive distinct sample compositions were systematically fabricated by progressively modulating the volumetric ratio of alumina to graphite reinforcement, as detailed in Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. The compositional series encompassed: pure alumina reinforcement (100% Al₂O₃-0% Gr, designated as S1), and four hybrid combinations incorporating increasing graphite content at 25% incremental intervals\u0026mdash;75% Al₂O₃-25% Gr (S2), 50% Al₂O₃-50% Gr (S3), 25% Al₂O₃-75% Gr (S4), and finally pure graphite reinforcement (0% Al₂O₃-100% Gr, designated as S5). This compositional strategy enabled systematic evaluation of how reinforcement phase proportion influences microstructural evolution and property development across the full compositional spectrum from monolithic ceramic to monolithic lubricant reinforcement.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab3\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eSample Designation and Compositional Specifications.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSample Code\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSample Designation\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eAl₂O₃ Content (vol%)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eGraphite Content (vol%)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eReinforcement Type\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eS1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003e100Al₂O₃\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e\u003cb\u003e100\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e\u003cb\u003e0\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u003cb\u003ePure ceramic\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eS2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003e75Al₂O₃-25Gr\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e\u003cb\u003e75\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e\u003cb\u003e25\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u003cb\u003eCeramic-dominant hybrid\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eS3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003e50Al₂O₃-50Gr\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e\u003cb\u003e50\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e\u003cb\u003e50\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u003cb\u003eBalanced hybrid\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eS4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003e25Al₂O₃-75Gr\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e\u003cb\u003e25\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e\u003cb\u003e75\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u003cb\u003eGraphite-dominant hybrid\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eS5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003e100Gr\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e\u003cb\u003e0\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e\u003cb\u003e100\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u003cb\u003ePure lubricant\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.3. Friction Stir Processing Procedure\u003c/h2\u003e \u003cp\u003eFriction stir processing was conducted on all composite samples employing a vertical milling machine equipped with a specially designed tool constructed from hardened H13 tool steel (60 HRC hardness). The tool geometry incorporated a shoulder with 18 millimeters diameter and a cylindrical pin with 2 millimeters diameter and 5.5 millimeters length, optimized for incorporation of reinforcing particles within the copper matrix. Prior to commencing processing operations, each substrate plate containing the machined groove filled with reinforcement powder (alumina and/or graphite) was securely clamped to prevent motion or deflection during the intense mechanical stirring action.\u003c/p\u003e \u003cp\u003eThe friction stir processing was executed using carefully controlled parameters selected through preliminary optimization trials to ensure optimal material consolidation and dynamic recrystallization while minimizing defect formation. Processing parameters maintained throughout the investigation comprised: rotational speed of 800 revolutions per minute (rpm), traverse speed (tool advance rate) of 50 millimeters per minute (mm/min), and axial downward force approximately 5\u0026ndash;6 kilonewtons. These parameters were deliberately held constant across all sample compositions to ensure that observed differences in microstructural and property outcomes resulted exclusively from compositional variations rather than processing condition modifications.\u003c/p\u003e \u003cp\u003eTwo-pass friction stir processing was employed for all samples to enhance particle distribution uniformity and ensure complete material consolidation. The first pass facilitated initial particle incorporation and mixing throughout the copper matrix, while the second pass\u0026mdash;conducted along the same processing path\u0026mdash;promoted further homogenization of the reinforcement distribution and refinement of the microstructure. Between successive passes, specimens were permitted to cool naturally to ambient temperature, preventing excessive heat accumulation that could promote grain coarsening or thermal degradation of the composite layer. The dual-pass strategy ensured superior particle dispersion compared to single-pass processing while avoiding the excessive grain growth sometimes associated with multi-pass operations involving three or more consecutive passes.\u003c/p\u003e \u003cp\u003eDuring each processing operation, the rotating tool generated intense localized plastic deformation and frictional heating within the stir zone, simultaneously stirring and mixing the reinforcement particles throughout the copper matrix. The mechanical action of the rotating pin promoted uniform particle distribution and facilitated development of robust metallurgical bonding between the reinforcement particles and the surrounding copper matrix. The friction stir processing operation proceeded unidirectionally along the entire 100-millimeter length of the substrate plate during each pass, with the tool geometry forcing material forward and generating characteristic flow patterns that promoted effective particle mixing.\u003c/p\u003e \u003cp\u003eFollowing completion of the two-pass friction stir processing sequence, all specimens were permitted to cool naturally to ambient temperature without artificial cooling intervention, allowing thermal stresses to relax gradually and preventing thermal shock-induced defect formation. This controlled processing protocol ensured reproducible results across all sample compositions while enabling systematic evaluation of how reinforcement phase composition influences the microstructural development and resultant mechanical and tribological properties of the Cu-Al₂O₃-Gr composite systems.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e2.4. Microstructural Characterization\u003c/h2\u003e \u003cp\u003eThe microstructural investigation of the FSP composites was performed using complementary optical and electron microscopy techniques to provide comprehensive documentation of compositional effects on material structure. Optical microscopy was employed initially to examine macroscopic features and overall distribution patterns characteristic of the processed composite layers. Optical micrographs were obtained using an Olympus SZH10 microscope with magnifications ranging from 50\u0026times; to 1000\u0026times;, providing progressive magnification levels suitable for visualization of both large-scale structural features and fine microstructural details.\u003c/p\u003e \u003cp\u003eSample preparation for optical microscopy examination followed established metallographic protocols. Specimens were sectioned perpendicular to the friction stir processing direction, mounted in epoxy resin, and subjected to sequential grinding and polishing operations employing progressively finer abrasive media to achieve a mirror-like surface finish. Following mechanical polishing, samples were etched chemically using a solution containing sulfuric and nitric acids, which selectively attacked grain boundaries and revealed the characteristic microstructural features developed during the two-pass friction stir processing operation. The etching procedure proved particularly effective in highlighting grain structure evolution, dynamic recrystallization phenomena, and the spatial distribution patterns of the reinforcement particles throughout the copper matrix.\u003c/p\u003e \u003cp\u003eHigh-resolution scanning electron microscopy (SEM) analysis was conducted to provide detailed microstructural documentation at substantially greater magnification levels than optical microscopy could achieve. A MIRA3 scanning electron microscope operated at an accelerating voltage of 15 kV provided magnification capabilities ranging from 500\u0026times; to 50,000\u0026times;, permitting visualization of fine microstructural details including individual particle distributions and interfacial characteristics between reinforcement phases and the copper matrix. The SEM technique proved particularly valuable for distinguishing microstructural variations across different zones generated during friction stir processing, including the stir zone (SZ), thermo-mechanically affected zone (TMAZ), and heat-affected zone (HAZ).\u003c/p\u003e \u003cp\u003eImaging within the SEM analysis employed two complementary contrast mechanisms to optimize information content. Secondary electron (SE) imaging provided enhanced visualization of surface topographical features and three-dimensional particle morphology, enabling clear observation of particle size variations and surface texture characteristics. Simultaneously, backscattered electron (BSE) imaging was employed to generate elemental contrast information, whereby variations in atomic number produced intensity variations that distinguished different phases and revealed particle boundaries within the composite microstructure. This dual-imaging approach ensured comprehensive documentation of both morphological features and compositional distinctions within the processed composite layers.\u003c/p\u003e \u003cp\u003eElemental composition analysis was performed through energy-dispersive X-ray spectroscopy (EDS) coupled directly to the SEM system, providing quantitative chemical information at spatially resolved locations within the processed specimens. Quantitative EDS analysis was systematically conducted at multiple locations within the stir zone of each processed sample to confirm alumina particle distribution throughout the copper matrix and detect potential elemental segregation or chemical interactions at particle-matrix interfaces that might influence property development. The EDS analysis was performed at an accelerating voltage of 15 kV, identical to the SEM imaging conditions, with data acquisition times of 60 seconds per analysis point to ensure acquisition of adequate signal intensity and statistical reliability of the resulting compositional determinations.\u003c/p\u003e \u003cp\u003eQuantitative grain size determination was performed employing digital image analysis software (Image J with MLI plugin) applied to high-magnification SEM micrographs of the stir zone microstructure. A minimum of five representative micrographs from the stir zone of each sample composition were analyzed to ensure statistical significance of the resulting grain size values. Individual grain diameter measurements were recorded from a minimum of 50 distinct grains per micrograph, generating a dataset comprising at least 250 individual grain measurements per sample composition. The mean linear intercept method was employed in accordance with ASTM E112 standards to calculate average grain diameter values, a methodology that provides standardized, reproducible grain size determinations enabling valid comparison across different sample compositions and processing conditions.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003e2.5. Microhardness Testing\u003c/h2\u003e \u003cp\u003eVickers microhardness measurements were systematically performed on all processed and baseline copper samples using an Akashi MVK-H21 microhardness tester to assess mechanical property development and compositional effects on hardness characteristics. All testing was conducted under rigidly standardized environmental conditions: ambient temperature of 22\u0026deg;C, relative humidity of 25%, and atmospheric pressure to ensure reproducibility and eliminate environmental variables that could influence measurement accuracy. The applied indentation load was maintained at 50 gram-force (gf), equivalent to 0.49 newtons, with an indentation dwell time of 15 seconds in accordance with ASTM E384 standards for microhardness testing procedures.\u003c/p\u003e \u003cp\u003eThe hardness evaluation protocol was deliberately designed to provide comprehensive characterization of mechanical property distribution throughout the processed composite microstructure. Initial measurements were performed on the unreinforced copper base metal to establish a baseline hardness reference value against which property enhancements could be quantified. Subsequently, indentations were performed systematically within the stir zone at regular intervals of 0.1 millimeter spacing perpendicular to the sample surface, progressing from the periphery toward the center of the composite layer. This spatially resolved sampling strategy permitted detailed characterization of hardness gradients, identification of localized property variations within the processed region, and assessment of the uniformity of reinforcement distribution effects on mechanical properties.\u003c/p\u003e \u003cp\u003eA minimum of three replicate indentations were conducted at each measurement location to establish statistical reliability of the resulting hardness values and quantify measurement variability. The average hardness value and standard deviation were subsequently calculated for each measurement location to provide both central tendency measures and dispersion quantification. Indentation spacing was deliberately maintained at minimum 2.5 times the diagonal indent dimension to prevent interaction effects between adjacent indents, which could artificially elevate or suppress hardness values at closely-spaced locations. All hardness values were recorded in Vickers hardness units (HV₀.₅) according to standardized conventions, with the notation reflecting the 50 gram-force test load employed. The resulting hardness values were systematically tabulated and graphically presented to establish comprehensive hardness profiles across the composite layer thickness, enabling clear visualization of property gradients and identification of processing-induced effects on material hardness.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003e2.6. Tribological Testing\u003c/h2\u003e \u003cp\u003eTribological evaluation of the processed composites was conducted using a WN1 pin-on-disc wear tester. Composite pin specimens were prepared with dimensions of 10 mm diameter and 5 mm height using wire-cut electrical discharge machining (EDM). All pin surfaces were polished using 1000-grit emery paper to eliminate surface irregularities before testing.\u003c/p\u003e \u003cp\u003eThe counterface disc was fabricated from 100Cr6 hardened steel (60 HRC) with dimensions of 50 mm diameter and 5 mm thickness. Prior to each test, the disc surface was cleaned magnetically and both pin and disc surfaces were cleaned with ethanol to remove contaminants. Wear testing parameters are summarized in Table\u0026nbsp;\u003cspan refid=\"Tab4\" class=\"InternalRef\"\u003e4\u003c/span\u003e. Testing was conducted at ambient temperature (25\u0026deg;C) with a total sliding distance of 1000 meters. A minimum of two to three replicate tests were performed for each sample composition, and friction coefficient was continuously recorded during testing.\u003c/p\u003e \u003cp\u003eWear volume loss was determined by measuring specimen mass before and after testing using a calibrated analytical balance (\u0026plusmn;\u0026thinsp;0.0001 g precision). Post-test wear surfaces were examined using SEM (magnification 500\u0026times; to 5000\u0026times;) to identify wear mechanisms. Energy-dispersive X-ray spectroscopy (EDS) analysis was performed on worn surfaces and wear debris to determine elemental composition and characterize tribological mechanisms.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab4\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 4\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003ePin-on-disc wear testing parameters.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"2\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTesting Parameter\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eValue\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eContact Surface Area (mm\u0026sup2;)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e25\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSliding Distance (m)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1000\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eApplied Normal Load (N)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSliding Velocity (m/s)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.5\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eDisc Material\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e100Cr6 Steel (60 HRC)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eEnvironmental Temperature (\u0026deg;C)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e20\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"3. Results and Discussion","content":"\u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003e3.1. Microstructural Characterization\u003c/h2\u003e \u003cp\u003eTo provide initial visualization of the structural characteristics of the friction stir processed composites containing varying graphite contents, optical stereomicroscopy examination of the sample surfaces was conducted at low magnification. Representative macroscopic images of all five composite compositions are presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e, which documents the surface appearance and overall processing-induced modifications visible at the optical microscopy scale. The friction stir processing operation was successfully executed across the complete range of compositional variations, with the uniform surface appearance indicating effective tool penetration and consistent material consolidation throughout the processing path.\u003c/p\u003e \u003cp\u003eExamination of the macroscopic images reveals progressive compositional-dependent color variations across the sample series. The 100% Al₂O₃ sample (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea) exhibits the characteristic light appearance typical of oxide ceramics, reflecting the dominant alumina phase distribution within the copper matrix. As graphite content increases progressively through the compositional series, the sample surfaces exhibit increasingly darker appearance in the 75Al₂O₃-25Gr (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb) and 50Al₂O₃-50Gr (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec) compositions, corresponding to increasing carbon content within the reinforcement phase ensemble. The graphite-dominant compositions (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ed and \u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ee) display distinctly darker surfaces reflecting the light-absorbing characteristics of the carbon-based lubricant phase.\u003c/p\u003e \u003cp\u003eThe macroscopic observations indicate that the two-pass friction stir processing successfully incorporated and distributed the reinforcement particles throughout the processed surface region across all compositional variants. The absence of visible processing defects, tunnel formations, or material voids in the macroscopic images suggests effective material consolidation and adequate tool-material interaction force balance during the processing operations. The uniform surface texture visible across the complete processing path in all samples indicates consistent tool advancement and stable processing conditions maintained throughout the friction stir processing procedure.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eOptical microscopy examination of the stir zone (SZ) microstructure was conducted to visualize grain structural evolution and characterize the microstructural transformations induced by two-pass friction stir processing. Representative optical micrographs from the stir zone of all five composite compositions are presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e, documenting the grain morphology and structural characteristics across the full compositional range from pure alumina to pure graphite reinforcement.\u003c/p\u003e \u003cp\u003eThe 100Al₂O₃ sample exhibits a refined grain structure characteristic of FSP-induced dynamic recrystallization processes. The optical micrographs reveal well-defined grain boundaries and predominantly equiaxed grain morphology throughout the stir zone, indicating successful grain boundary refinement through active DRX mechanisms. This grain refinement is attributed to continuous dynamic recrystallization (CDRX) and geometric dynamic recrystallization (GDRX) phenomena promoted by the intense thermomechanical deformation generated during friction stir processing [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eProgressive compositional variation toward increasing graphite content reveals interesting microstructural responses. The intermediate compositions (75Al₂O₃-25Gr and 50Al₂O₃-50Gr) demonstrate comparable grain structures with well-defined boundaries and equiaxed morphologies, indicating that moderate graphite incorporation does not significantly disrupt the recrystallization mechanisms characteristic of alumina-reinforced composites. However, subtle microstructural refinement becomes increasingly evident as graphite content increases, suggesting compositional-dependent effects on grain evolution.\u003c/p\u003e \u003cp\u003eThe graphite-enriched compositions (25Al₂O₃-75Gr and 100Gr) display distinctly modified microstructural characteristics. The pure graphite sample (100Gr) exhibits noticeably refined grain structure throughout the stir zone, with grain boundaries appearing sharper and grain morphology remaining equiaxed. This pronounced grain refinement in the graphite-dominant composition is attributed to Zener pinning mechanisms [\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e], whereby graphite particles effectively lock grain boundaries and constrain recrystallized grain growth, preventing excessive grain coarsening and maintaining fine microstructural dimensions.\u003c/p\u003e \u003cp\u003eThe consistent presence of well-defined grain boundaries across all compositions indicates successful dynamic recrystallization during the two-pass FSP operation. The overall trend suggests that reinforcement particle content, particularly graphite, plays an important role in microstructural refinement through grain boundary pinning mechanisms [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eExamination of the thermo-mechanically affected zone (TMAZ) microstructure provides important insights into the material behavior at the transition region between the intensely processed stir zone and the thermally affected base material. Representative SEM micrographs of the TMAZ region from all five composite compositions are presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e, documenting the characteristic microstructural features and compositional-dependent variations across this critical transitional region.\u003c/p\u003e \u003cp\u003eThe TMAZ region exhibits distinctly different microstructural characteristics compared to the stir zone, reflecting the reduced mechanical deformation and thermal input experienced in this peripheral zone during friction stir processing. In the 100Al₂O₃ composition (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea), the TMAZ displays evidence of partial dynamic recrystallization with mixed grain populations including both recrystallized and partially recovered substructures. Alumina particles are present but distributed more heterogeneously than in the stir zone, reflecting the reduced stirring intensity at this transitional location.\u003c/p\u003e \u003cp\u003eThe intermediate compositions (75Al₂O₃-25Gr and 50Al₂O₃-50Gr) demonstrate progressive microstructural refinement in the TMAZ region (Figs.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb and \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ec). The hybrid reinforcement phases create complex interactions with the developing substructure, promoting partial grain refinement even in this lower-deformation zone. Particle distribution becomes increasingly variable, with evidence of localized clustering in certain regions while other areas exhibit relatively uniform dispersion.\u003c/p\u003e \u003cp\u003eThe graphite-enriched compositions (25Al₂O₃-75Gr and 100Gr) show distinctly modified TMAZ microstructure (Figs.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ed and \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ee). The abundant graphite particles effectively constrain microstructural evolution through Zener pinning mechanisms, resulting in preserved subgrain structures and refined boundary networks. The TMAZ in the pure graphite sample (100Gr) exhibits the finest apparent substructure among all compositions, indicating that graphite particle density plays a dominant role in controlling microstructural coarsening tendencies even in lower-deformation regions.\u003c/p\u003e \u003cp\u003eThe progressive transition from fully recrystallized stir zone structure to partially recovered TMAZ structure is clearly visible across all samples, with reinforcement particle content modulating the extent of microstructural preservation. These observations indicate that reinforcement particles, particularly graphite, exert significant influence on thermomechanical microstructural evolution throughout the friction stir processed region [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eGrain size distribution analysis of the stir zone microstructure was performed using digital image analysis software applied to high-magnification micrographs. The frequency histograms and statistical analysis of grain size distributions across all composite compositions are presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e, providing quantitative documentation of compositional effects on grain refinement. Complementary bar chart analysis (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e, subplot d) compares the average grain diameter values across the five sample compositions, enabling direct evaluation of compositional-dependent grain size variations.\u003c/p\u003e \u003cp\u003eThe 100Al₂O₃ sample exhibits a relatively broad grain size distribution with grain diameters extending across a considerable range. The grain size histogram shows a distribution centered in the intermediate range with notable frequency at smaller grain dimensions, indicating predominant development of fine recrystallized grains during the two-pass friction stir processing operation. This distribution pattern is characteristic of successful dynamic recrystallization wherein repeated mechanical deformation and thermal cycling during two passes promotes formation of refined equiaxed grains.\u003c/p\u003e \u003cp\u003eProgressive incorporation of graphite into the alumina matrix produces systematic changes in grain size distribution characteristics. The intermediate compositions (75Al₂O₃-25Gr and 50Al₂O₃-50Gr) demonstrate grain size distributions broadly comparable to the pure alumina sample, with grain diameters distributed throughout the intermediate size range. However, subtle leftward shifts in the distribution histograms suggest modest grain refinement accompanying graphite incorporation, indicating that reinforcement particles actively participate in grain boundary pinning mechanisms.\u003c/p\u003e \u003cp\u003eThe graphite-enriched compositions (25Al₂O₃-75Gr and particularly 100Gr) demonstrate distinctly narrower grain size distributions concentrated at smaller grain dimensions. The 100Gr composition exhibits the most pronounced refinement, with the histogram displaying a sharp, narrow peak concentrated at lower grain diameter values. This marked grain size reduction compared to alumina-dominant compositions directly reflects the enhanced Zener pinning effectiveness of graphite particles, whose abundance constrains grain growth and maintains exceptionally fine microstructural dimensions throughout the stir zone.\u003c/p\u003e \u003cp\u003eThe quantitative comparison presented in the bar chart (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ed) reveals a non-monotonic relationship between graphite content and average grain size, with optimal grain refinement achieved in the pure graphite sample. This compositional dependence underscores the importance of particle density and type in controlling microstructural evolution during friction stir processing, demonstrating that graphite particles provide superior grain boundary pinning compared to alumina particles at equivalent volumetric loadings [\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eDetailed microstructural examination of the stir zone was conducted using high-resolution scanning electron microscopy (SEM) combined with energy-dispersive X-ray spectroscopy (EDS) to characterize particle distribution, reinforcement incorporation, and elemental composition at spatially resolved locations. Representative backscattered electron (BSE) micrographs with marked analysis points and corresponding EDS spectral data from all five composite compositions are presented in Figs.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e and \u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e.\u003c/p\u003e \u003cp\u003eThe 100Al₂O₃ sample exhibits predominantly copper matrix regions interspersed with well-distributed alumina particles appearing as bright contrasting phases in backscattered electron imaging. The microstructure displays relatively sparse particle distribution reflecting the baseline reinforcement density. Although detailed point analysis was not performed on this reference composition, the SEM observation confirms successful particle incorporation into the copper matrix through the friction stir processing operation.\u003c/p\u003e \u003cp\u003eThe 75Al₂O₃-25Gr sample (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003eb) displays increased particle density with both alumina (bright contrast) and graphite (dark contrast) phases distributed throughout the copper matrix. Point analysis A in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003eb yields EDS spectrum 8a confirming strong aluminum and oxygen peaks characteristic of alumina particles embedded in the copper matrix. Analysis point B (EDS spectrum 8b) reveals similar alumina-dominant composition with characteristic Al, O, and Cu signals. Point C (EDS spectrum 8c) demonstrates mixed elemental signals indicating analysis of regions containing both reinforcement phases and copper matrix material. The spatial distribution pattern demonstrates effective mechanical stirring during two-pass friction stir processing, with reinforcement particles well-distributed throughout the processed layer.\u003c/p\u003e \u003cp\u003eThe balanced 50Al₂O₃-50Gr composition (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ec) exhibits noticeably increased particle density reflecting the cumulative volumetric contribution of both reinforcement phases at equal proportions. Point analysis A (EDS spectrum 8d) confirms alumina composition with dominant Al and O peaks. Analysis point B (EDS spectrum 8e) reveals transition region composition with contributions from both matrix and reinforcement phases. The increased particle density compared to the 75Al₂O₃-25Gr sample is evident from the more frequent contrasting particles visible in the backscattered electron micrographs.\u003c/p\u003e \u003cp\u003eThe 25Al₂O₃-75Gr sample (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ed) demonstrates dramatically increased particle density with prominent dark-contrasting graphite particles dominating visual appearance alongside subordinate bright alumina phases. Point analysis A (EDS spectrum 8f) reveals matrix-dominated composition with strong copper signals and lower reinforcement phase contributions. Analysis point B (EDS spectrum 8g) shows aluminum and oxygen enrichment indicating alumina particle location. Point C (EDS spectrum 8h) confirms carbon-enriched composition corresponding to graphite particle region, with dominant carbon peak clearly distinguishing the lubricant phase composition. The multiple-phase detection across adjacent analysis points underscores the complex microstructural heterogeneity characteristic of balanced hybrid reinforcement systems.\u003c/p\u003e \u003cp\u003eThe pure graphite sample (100Gr) (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ee) exhibits the most extensive particle distribution with dense accumulation of dark-contrasting graphite particles throughout the processed layer. Point analysis A (EDS spectrum 8i) confirms overwhelming carbon enrichment characteristic of the exclusive graphite reinforcement, with strong carbon peak prominence and subordinate copper matrix contributions. Despite this extraordinarily high particle density, the microstructure does not exhibit visible void formation or agglomeration clustering, indicating that two-pass friction stir processing successfully distributed even maximal graphite loadings throughout the copper matrix without generating deleterious segregation defects [\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eProgressive examination of EDS spectra across the entire compositional series reveals systematic variations in elemental distribution consistent with increasing graphite content. The alumina-dominant compositions display prominent aluminum and oxygen signals, while graphite-enriched samples show progressively dominant carbon contributions. All samples exhibit strong copper signals confirming continuous matrix connectivity throughout the processed layers, indicating absence of complete particle-matrix separation or extensive debonding failures. The consistent presence of well-integrated particles without visible gap formation at particle-matrix interfaces suggests effective metallurgical bonding development during thermomechanical processing, reflecting successful interfacial contact establishment necessary for load transfer functionality.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eDetailed quantitative analysis of reinforcement particle size distributions within the stir zone was conducted through systematic examination of high-magnification SEM micrographs. The frequency histograms documenting particle size distributions for all five composite compositions are presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e (subplots a-e), while a comparative bar chart (Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003ef) provides direct comparison of average particle size across the sample series.\u003c/p\u003e \u003cp\u003eThe 100Al₂O₃ sample exhibits a relatively broad particle size distribution extending across a considerable dimensional range. The frequency histogram reveals a distribution with multiple peaks suggesting heterogeneous particle dimensions, reflecting the natural particle size variation inherent to the as-received alumina powder. Despite this initial size heterogeneity, the particles maintain predominantly fine dimensions consistent with effective reinforcement incorporation during friction stir processing.\u003c/p\u003e \u003cp\u003eSystematic introduction of graphite reinforcement into the composite system produces notable modifications in overall particle size distribution characteristics. The 75Al₂O₃-25Gr composition demonstrates particle size distribution encompassing contributions from both alumina and graphite phases. The histogram reveals two distinct population regions, likely reflecting the inherent size differences between alumina (fine, submicron to low-micrometer range) and graphite (larger, 50\u0026ndash;100 micrometer range) particles employed in composite fabrication. This dual-population distribution reflects the bimodal particle population introduced through the reinforcement strategy.\u003c/p\u003e \u003cp\u003eThe 50Al₂O₃-50Gr balanced composition similarly exhibits bimodal particle size characteristics reflecting equivalent proportions of both reinforcement phases. The distribution histogram displays comparable peak regions to the 75Al₂O₃-25Gr composition, with the balanced composition transition point demonstrating no dramatic shifts in overall particle population characteristics, suggesting relatively consistent particle sizing from both reinforcement sources.\u003c/p\u003e \u003cp\u003eThe 25Al₂O₃-75Gr sample exhibits particle size distribution progressively dominated by graphite-scale dimensions reflecting the compositional shift toward graphite enrichment. The histogram displays a major distribution peak at larger particle sizes corresponding to graphite particle dimensions, with subordinate contributions from residual alumina phases. The shift toward larger average particle size becomes evident from the rightward distribution displacement compared to earlier compositions.\u003c/p\u003e \u003cp\u003eThe pure graphite sample (100Gr) demonstrates the most distinctive particle size distribution profile, with strong concentration at larger particle dimensions characteristic of the graphite powder specification. The narrow, well-defined distribution peak indicates relatively consistent graphite particle sizing throughout the processed composite, reflecting the uniform particle size range provided by the graphite powder source. Despite this relatively large particle scale compared to alumina, the graphite particles achieved effective distribution throughout the copper matrix without generating problematic segregation or clustering defects.\u003c/p\u003e \u003cp\u003eThe quantitative bar chart comparison (Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003ef) reveals systematic trends in average particle size across the compositional series. The alumina-based composition displays fine average particle dimensions reflecting the submicron to low-micrometer alumina particle scale. Progressive graphite incorporation produces compositional-dependent increases in average particle size, reflecting the larger characteristic dimensions of graphite powder compared to alumina. The pure graphite sample exhibits the largest average particle size reflecting exclusive graphite reinforcement at its specified 50\u0026ndash;100 micrometer dimensional range. These particle size trends directly correlate with the microstructural refinement patterns observed in earlier sections, where fine particle distributions facilitate more effective grain boundary pinning and microstructural control compared to coarser reinforcement particles [\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e, \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003e3.2. Hardness Results\u003c/h2\u003e \u003cp\u003eVickers microhardness measurements were systematically performed across the cross-sectional profile of all processed samples, spanning from the stir zone center toward the peripheral regions to characterize hardness development and property gradients induced by compositional variation. Hardness profiles across the processed layer thickness for all five composite compositions are presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e, documenting spatial hardness distribution and comparative mechanical property enhancements.\u003c/p\u003e \u003cp\u003eThe unprocessed base copper material exhibits characteristic low hardness values around 60 HV, reflecting the inherently soft nature of commercially pure copper. In stark contrast, the 100Al₂O₃ composite displays substantial hardness elevation throughout the stir zone with values reaching approximately 110\u0026ndash;115 HV in the central processed region. This dramatic hardness increase directly correlates with the microstructural refinement documented in earlier microscopy sections, reflecting the combined strengthening contributions from grain size reduction via dynamic recrystallization and load transfer mechanisms to the hard alumina reinforcement particles. The hardness profile maintains relatively consistent values across the stir zone width before declining gradually toward the TMAZ and base material regions, demonstrating localized property enhancement restricted to the thermomechanically processed zone.\u003c/p\u003e \u003cp\u003eThe progressive incorporation of graphite into the alumina matrix produces compositionally-dependent modifications of hardness characteristics. The 75Al₂O₃-25Gr composition exhibits hardness values reaching approximately 120 HV within the stir zone center, representing modest elevation compared to the pure alumina reference. The graphite introduction contributes to overall hardness maintenance while moderating the property gradient profile. The hardness profile displays a relatively flat distribution across the stir zone width, indicating uniform reinforcement particle distribution and consistent strengthening mechanisms throughout the processed region.\u003c/p\u003e \u003cp\u003eThe 50Al₂O₃-50Gr balanced composition demonstrates hardness elevation to approximately 120\u0026ndash;125 HV, achieving among the highest absolute hardness values within the compositional series. This optimal hardness response at intermediate composition reflects synergistic interactions between alumina and graphite reinforcement phases. The alumina particles provide direct strengthening through ceramic hardness and grain boundary pinning, while graphite maintains fine microstructural dimensions through enhanced Zener pinning effectiveness, collectively producing hardness values exceeding those achieved by monolithic reinforcement systems. The hardness profile remains consistently elevated across the stir zone, indicating effective property homogenization through dual-phase reinforcement [\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThe 25Al₂O₃-75Gr composition exhibits hardness values around 95\u0026ndash;100 HV within the stir zone, representing modest hardness compared to the balanced composition but substantial improvement over the unprocessed base material. The reduced hardness compared to alumina-dominant compositions reflects the inherently lower intrinsic hardness of graphite particles compared to ceramic alumina, despite the superior grain boundary pinning effectiveness of graphite. The hardness profile remains relatively constant across the stir zone, suggesting effective particle distribution maintains uniform mechanical properties throughout the processed region.\u003c/p\u003e \u003cp\u003eThe pure graphite sample (100Gr) demonstrates sustained hardness elevation approaching 130\u0026ndash;140 HV within the stir zone center\u0026mdash;the maximum hardness observed across the entire compositional series. This remarkable hardness achievement in the absence of ceramic reinforcement represents a significant finding, attributing the exceptional hardness development to extraordinarily fine grain structures achieved through effective graphite particle Zener pinning of grain boundaries. The hardness profile displays the flattest characteristic among all compositions, indicating the most uniform property distribution and most effective grain boundary constraint provided by the highest graphite particle density.\u003c/p\u003e \u003cp\u003eThe hardness evolution across the compositional series exhibits clear correspondence with microstructural observations documented in preceding sections. The exceptional hardness of the 100Gr sample directly correlates with the finest observed grain structures in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e, confirming the dominant role of grain size reduction in determining hardness through Hall-Petch relationships [\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e]. The intermediate hardness maximum observed at the 50Al₂O₃-50Gr composition reflects the optimal balance between ceramic phase strengthening contributions and graphite Zener pinning effectiveness, creating composite conditions superior to either single-reinforcement system. The uniform hardness profiles characteristic of hybrid compositions reflects the homogeneous reinforcement particle distributions documented through SEM examination, demonstrating the effectiveness of two-pass friction stir processing in generating consistent property distribution throughout the processed layers.\u003c/p\u003e \u003cp\u003eThe sharp hardness decline observed at the TMAZ-SZ boundary in all samples corresponds to the transition from fully recrystallized stir zone structure to partially recovered thermo-mechanically affected zone microstructure documented in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e, establishing quantitative mechanical property verification of the qualitative microstructural transitions. This consistent hardness gradient pattern across all compositions confirms that the measured property distributions directly reflect underlying microstructural variations, providing mechanistic explanation for observed hardness trends through established strengthening mechanisms of grain refinement and particle-matrix interactions.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section2\"\u003e \u003ch2\u003e3.3. Tribological Performance\u003c/h2\u003e \u003cp\u003eComprehensive tribological evaluation was conducted through standardized pin-on-disc wear testing to characterize compositional effects on friction and wear resistance. The tribological behavior is documented through four complementary analyses presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e: cumulative weight loss evolution (Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003ea), calculated wear rates (Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003eb), real-time friction coefficient recordings (Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003ec), and comparative friction coefficient values (Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003ed).\u003c/p\u003e \u003cp\u003eThe cumulative weight loss curves (Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003ea) reveal dramatic compositional-dependent wear behavior throughout the 1000-meter sliding distance. The unprocessed base copper exhibits the most severe wear with continuous linear weight loss accumulation, reflecting the inherently poor wear resistance of soft copper under abrasive sliding contact. The 100Al₂O₃ sample demonstrates substantial wear reduction compared to base copper, with cumulative weight loss decreasing by approximately 40\u0026ndash;50%, directly attributable to the enhanced hardness documented in Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e and the load-bearing capacity provided by hard alumina particles.\u003c/p\u003e \u003cp\u003eProgressive graphite incorporation produces systematic improvements in wear resistance. The 75Al₂O₃-25Gr composition exhibits modest wear reduction compared to pure alumina, while the 50Al₂O₃-50Gr balanced composition demonstrates dramatically superior wear resistance with cumulative weight loss reduced to approximately one-third that of the alumina-only sample. This exceptional performance directly correlates with the synergistic microstructural refinement documented in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e and the optimal hardness achieved in this composition (Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e), demonstrating that balanced reinforcement proportions deliver superior tribological performance through combined ceramic strengthening and graphite lubrication mechanisms [\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThe graphite-enriched compositions maintain excellent wear resistance throughout the sliding distance. The 25Al₂O₃-75Gr sample exhibits weight loss comparable to the balanced composition, while the pure graphite sample (100Gr) achieves the absolute minimum cumulative weight loss across the entire compositional series\u0026mdash;approximately 80% reduction compared to base copper and 70% reduction compared to pure alumina. This remarkable wear resistance achievement despite the absence of hard ceramic reinforcement represents a significant finding, establishing that self-lubricating mechanisms and fine microstructural dimensions can collectively deliver wear performance exceeding that provided by conventional ceramic reinforcement strategies.\u003c/p\u003e \u003cp\u003eThe calculated wear rates (Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003eb) provide quantitative verification of compositional effects on material removal rates. The base copper exhibits the maximum wear rate reflecting its soft matrix and absence of protective reinforcement phases. The 100Al₂O₃ composition shows substantial wear rate reduction, while progressive graphite incorporation produces systematic wear rate decreases reaching minimum values in the graphite-enriched compositions. The 50Al₂O₃-50Gr balanced composition and pure graphite sample exhibit comparable wear rates\u0026mdash;both approximately 15\u0026ndash;20% of the base copper value\u0026mdash;confirming that either balanced hybrid reinforcement or exclusive graphite reinforcement delivers exceptional wear resistance through distinct but equally effective mechanisms.\u003c/p\u003e \u003cp\u003eThese wear rate trends establish direct correlations with microstructural observations documented throughout Section 3. The finest grain structures observed in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e for the 100Gr sample translate directly into exceptional wear resistance through enhanced resistance to plastic deformation and subsurface damage accumulation. Similarly, the elevated hardness values documented in Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e for both the balanced composition and pure graphite sample provide mechanical resistance against abrasive penetration, reducing material removal rates during sliding contact.\u003c/p\u003e \u003cp\u003eThe real-time friction coefficient recordings (Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003ec) document tribological behavior evolution throughout the wear testing duration, revealing compositional-dependent friction characteristics and transient response patterns. The base copper exhibits relatively high and unstable friction coefficient with significant fluctuations throughout testing, reflecting continuous surface damage and absence of protective lubricating films. The 100Al₂O₃ sample demonstrates modestly reduced friction coefficient with improved stability, indicating that ceramic reinforcement provides some surface protection but lacks intrinsic lubrication capabilities.\u003c/p\u003e \u003cp\u003eProgressive graphite incorporation produces systematic friction coefficient reductions and enhanced stability. The intermediate compositions (75Al₂O₃-25Gr and 50Al₂O₃-50Gr) display progressively lower friction values with diminished fluctuation amplitude, indicating development of protective mechanically mixed layers (MML) containing graphite particles that provide solid-state lubrication during sliding contact. The graphite-enriched compositions demonstrate the most stable friction coefficient behavior with minimal fluctuations, reflecting continuous lubrication provided by abundant graphite particles transferred to the counterface disc surface.\u003c/p\u003e \u003cp\u003eThe pure graphite sample (100Gr) exhibits the lowest and most stable friction coefficient across the entire testing duration\u0026mdash;approximately 0.35\u0026ndash;0.40 compared to 0.60\u0026ndash;0.65 for base copper\u0026mdash;representing approximately 40% friction reduction. This dramatic friction suppression directly results from the self-lubricating graphite particles documented in the SEM analyses (Figs.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e\u0026ndash;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e), which form protective tribofilms on both pin and counterface surfaces, effectively separating metal-metal contact and providing continuous solid lubrication throughout the sliding duration.\u003c/p\u003e \u003cp\u003eThe averaged friction coefficient comparison (Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003ed) provides quantitative documentation of compositional effects on steady-state friction behavior. Progressive graphite incorporation produces monotonic friction coefficient reduction, with the pure graphite sample achieving minimum friction values reflecting optimal self-lubricating capacity. These friction trends correlate directly with particle distribution characteristics documented in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e, where graphite particle abundance increases systematically with composition, providing progressively greater lubrication functionality [\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThe exceptional tribological performance of graphite-enriched compositions, particularly the 100Gr sample, establishes remarkable correlations with microstructural characteristics documented throughout this investigation. The finest grain structures observed in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e for the 100Gr composition provide enhanced resistance to subsurface plastic deformation during wear contact, preventing excessive material displacement and crack propagation that would accelerate material removal. The elevated hardness documented in Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e for this composition\u0026mdash;despite the absence of hard ceramic particles\u0026mdash;reflects the grain refinement effectiveness of graphite Zener pinning, translating microstructural refinement directly into mechanical property enhancement and wear resistance improvement.\u003c/p\u003e \u003cp\u003eThe particle distribution characteristics documented in Figs.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e\u0026ndash;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e reveal abundant graphite particles homogeneously distributed throughout the copper matrix in the 100Gr sample, providing continuous solid lubrication sources throughout the wear surface. During sliding contact, these graphite particles transfer to the counterface steel disc, forming protective tribofilms that reduce direct metal-metal contact and suppress adhesive wear mechanisms. Simultaneously, the layered crystal structure of graphite particles facilitates easy shear along basal planes, accommodating sliding displacement without generating high friction forces or excessive material removal [\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThe tribological superiority of the 50Al₂O₃-50Gr balanced composition similarly reflects synergistic interactions between reinforcement phases. The alumina particles provide load-bearing capacity and subsurface strengthening while graphite particles deliver surface lubrication, collectively creating conditions where both wear resistance mechanisms\u0026mdash;mechanical hardness and chemical lubrication\u0026mdash;operate simultaneously. This balanced reinforcement strategy delivers tribological performance approaching that of the pure graphite sample while maintaining higher absolute hardness values that could prove advantageous under more severe loading conditions.\u003c/p\u003e \u003cp\u003ePost-test wear surface examination through scanning electron microscopy provides macroscopic visualization of wear scar characteristics and the distinct wear mechanisms operative during sliding contact across the compositional series. Representative SEM micrographs of worn pin surfaces after 1000 meters of sliding distance are presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003e, documenting the compositional-dependent evolution of wear mechanisms and surface damage patterns reflecting the microstructural and mechanical property differences established earlier.\u003c/p\u003e \u003cp\u003eThe 100Al₂O₃ sample (Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003ea) exhibits well-organized wear scar morphology with clearly visible parallel wear tracks aligned along the sliding direction. The wear surface displays characteristic abrasive wear topography with groove patterns indicating controlled material removal through interaction with counterface asperities. Multiple fine scratches and ordered furrows document systematic surface degradation reflecting the hard alumina particles' resistance to plastic deformation. The organized surface features without evidence of extensive ploughing or irregular material displacement indicate that abrasive mechanisms dominate the wear behavior, with the ceramic particles providing adequate surface constraint and hardness to direct material removal through predictable abrasion rather than chaotic adhesive processes.\u003c/p\u003e \u003cp\u003eThe 75Al₂O₃-25Gr sample (Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003eb) demonstrates progressive transition toward smoother wear surface characteristics, with noticeably diminished groove amplitude and reduced surface roughness compared to pure alumina. The emergence of flatter surface regions suggests partial suppression of deep abrasive scratching through protective effects of transferred graphite particles. The wear track morphology becomes less pronounced, indicating that graphite lubrication effects begin modulating the tribological interaction, reducing the severity of asperity-surface contact and moderating material removal mechanisms.\u003c/p\u003e \u003cp\u003eThe 50Al₂O₃-50Gr balanced composition (Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003ec) displays dramatically improved wear surface characteristics with the smoothest and most organized topography among the hybrid compositions. The wear surface exhibits extensively developed parallel wear tracks with exceptionally refined groove features and minimal evidence of irregular material displacement. The controlled, linear wear track patterns reflect systematic material removal through mild wear mechanisms, indicating effective suppression of aggressive abrasive and adhesive processes through the synergistic combination of ceramic reinforcement and graphite-generated lubrication. The surface topology directly correlates with the exceptional tribological performance documented quantitatively in Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e, confirming that balanced reinforcement composition creates optimal conditions for wear resistance.\u003c/p\u003e \u003cp\u003eThe 25Al₂O₃-75Gr composition (Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003ed) exhibits noticeably smoother wear surface compared to earlier compositions, with barely perceptible wear track definition and minimal surface roughness. The transition toward graphite-dominant lubrication becomes evident through the dramatically reduced surface damage features and emergence of exceptionally smooth topography reflecting the protective tribofilm effects of abundant graphite particles. The wear mechanism clearly transitions from abrasive dominance toward surface lubrication control [\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThe pure graphite sample (100Gr) (Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003ee) presents the most remarkable wear surface morphology, displaying near-mirror-like polish with essentially imperceptible wear track features and minimal visible surface degradation. The wear surface exhibits characteristics of boundary lubrication dominance, where graphite tribofilm formation effectively decouples the tribological response from bulk material characteristics and prevents direct metal-to-metal contact throughout the sliding duration. The exceptional surface smoothness, remarkable given the severe 1000-meter sliding distance, establishes that continuous graphite lubrication mechanisms can suppress material removal rates to minimal levels through protective boundary layer maintenance.\u003c/p\u003e \u003cp\u003eThe systematic wear surface evolution across the compositional series documents distinct mechanistic transitions reflecting the changing balance between ceramic hardness and graphite lubrication contributions. The pure alumina composite operates under abrasive wear conditions where hard particles constrain material removal to predictable patterns reflecting surface interaction geometry. Introduction of graphite modifies this strictly mechanical paradigm through physical and chemical lubrication mechanisms inherent to graphite's layered crystal structure.\u003c/p\u003e \u003cp\u003eThe progression from organized abrasive scratching patterns in alumina-dominant samples to increasingly smooth surfaces in graphite-enriched materials reflects progressive establishment of protective tribofilms that isolate bulk material from direct contact damage. The balanced composition achieves remarkable wear surface smoothness through simultaneous optimization of mechanical resistance and surface lubrication, while the pure graphite sample demonstrates that self-lubricating mechanisms alone can achieve exceptional tribological performance through continuous protective boundary layer formation [\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThe correlation between wear surface morphology and quantitative metrics presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e validates the mechanistic interpretations derived from qualitative SEM observation. The lowest wear rates and friction coefficients observed for graphite-enriched compositions directly correspond to the smoothest wear surfaces, confirming that effective lubrication establishment through graphite particle abundance provides superior wear protection compared to purely mechanical strengthening strategies. The marked difference between the organized but still-damaged surfaces characteristic of pure alumina and the essentially undamaged surfaces of graphite-enriched samples establishes that self-lubrication mechanisms fundamentally alter tribological interaction patterns, preventing the surface damage progression that would inevitably accumulate through extended sliding contact under mechanically-dominated conditions.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eComprehensive analysis of wear debris generated during tribological testing provides mechanistic insights into material removal processes and particle interactions at the sliding interface. Scanning electron microscopy examination of wear debris particles collected from the tribological test environment documents the compositional-dependent characteristics of materials removed during sliding contact. Representative SEM micrographs of wear debris particles from all five composite compositions are presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e13\u003c/span\u003e, documenting particle morphology, size distributions, and compositional signatures reflecting the wear mechanisms operative in each material system.\u003c/p\u003e \u003cp\u003eThe wear debris from the 100Al₂O₃ sample (Fig.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e13\u003c/span\u003ea) consists predominantly of fine, irregular aluminum oxide particles ranging from submicron dimensions to several micrometers, reflecting fragmentation of alumina particles through contact stress concentration and mechanically mixed layer formation. The particles display sharp, angular morphologies characteristic of brittle ceramic fracture, indicating that alumina particles undergo direct comminution through impact loading and shear stress concentration during sliding contact. The fine particulate nature and abundant debris generation reflect active material removal through abrasive wear mechanisms, consistent with the organized wear surface patterns documented in Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003ea.\u003c/p\u003e \u003cp\u003eThe wear debris from the 75Al₂O₃-25Gr composition (Fig.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e13\u003c/span\u003eb) displays mixed particle populations reflecting contributions from both alumina and graphite reinforcement phases. Angular aluminum oxide fragments coexist with larger flake-like graphite particles displaying characteristic layered morphology. The graphite particles appear relatively intact despite the mechanical degradation, reflecting the layered structure's capacity to accommodate shear displacement without fracture fragmentation.\u003c/p\u003e \u003cp\u003eThe 50Al₂O₃-50Gr balanced composition (Fig.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e13\u003c/span\u003ec) generates wear debris with notably increased graphite particle abundance and decreased alumina fragmentation compared to the graphite-limited compositions. The predominantly flake-like graphite particles reflect easy mechanical separation along basal planes under shear conditions, while residual alumina particles remain relatively large compared to debris from pure alumina samples. This debris composition change directly correlates with the reduced wear rates documented in Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003eb, indicating that graphite particle abundance reduces overall material removal rates through protective tribofilm formation that moderates stress concentration on ceramic particles.\u003c/p\u003e \u003cp\u003eThe 25Al₂O₃-75Gr composition (Fig.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e13\u003c/span\u003ed) yields wear debris dominated by graphite flakes with minimal alumina fragmentation, reflecting the predominant role of graphite in controlling tribological processes at high graphite content. The graphite debris particles display the characteristic sheet-like morphology reflective of mechanical exfoliation and transfer to generate protective boundary layers [\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThe pure graphite sample (100Gr) (Fig.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e13\u003c/span\u003ee) generates wear debris consisting almost exclusively of graphite particles displaying the distinctive flake morphology and layered structure characteristic of mechanically exfoliated graphite. The remarkable scarcity of matrix copper particles in the debris field indicates that protective graphite-enriched boundary layers effectively shield the underlying copper matrix from direct contact and material removal. The predominance of graphite particles over copper matrix material in the debris despite the graphite phase comprising the minor volumetric component within the composite reflects the preferential material removal of the self-lubricating phase and its efficient transfer to generate protective surface conditions.\u003c/p\u003e \u003cp\u003eThe systematic analysis of wear debris particle sizes documented in Fig.\u0026nbsp;\u003cspan refid=\"Fig14\" class=\"InternalRef\"\u003e14\u003c/span\u003e reveals distinct compositional-dependent trends in wear particle generation. The pure alumina sample (S1) generates the finest average particle sizes reflecting acute fragmentation of ceramic particles through abrasive and brittle fracture mechanisms. Progressive graphite incorporation produces progressively larger average debris particle sizes, reflecting the transition from fine comminution of brittle ceramics toward coarser mechanical exfoliation of layered graphite particles. The pure graphite sample (S5) achieves the largest average wear debris particle size, indicating that graphite particles release from the composite as intact or partially fragmented pieces through low-energy mechanical separation along existing layer boundaries rather than through high-stress brittle fracture.\u003c/p\u003e \u003cp\u003eThe compositional-dependent evolution of wear debris characteristics documents the fundamental transition in material removal mechanisms across the compositional series. The fine, angular debris characteristic of pure alumina reflects stress-concentrated brittle fracture generating abundant small particles that contribute significantly to abrasive wear acceleration. The transition toward graphite-dominated debris in enriched compositions reflects suppression of ceramic fragmentation through protective tribofilm establishment and consequent reduction in direct contact stress on alumina particles [\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThe exceptional abundance of intact graphite particles in the pure graphite composite debris, despite their removal from the bulk material, reflects preferential transfer of graphite toward the tribological interface where they form protective boundary layers. This preferential transfer mechanism effectively concentrates self-lubricating material precisely at the location where tribological protection is most critically needed, creating a self-reinforcing tribological process wherein material removal rate reduction is accompanied by enhanced protective boundary layer development. The paradoxical observation that the lowest wear rate sample (100Gr) generates the largest debris particles\u0026mdash;seemingly contradicting conventional wear theory\u0026mdash;reflects the fundamental mechanism shift from stress-driven fragmentation toward preferential phase transfer, where larger but less abundant particles indicate effective protective layer formation with minimal bulk material removal.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"4. Conclusion","content":"\u003cp\u003eTwo-pass friction stir processing of copper substrates with varying proportions of alumina and graphite reinforcement successfully fabricated hybrid metal matrix composites with systematically engineered microstructural, mechanical, and tribological properties. The investigation systematically modulated the alumina-to-graphite ratio from 100% Al₂O₃ through intermediate compositions to 100% Gr, establishing quantitative relationships between reinforcement composition and resulting material characteristics.\u003c/p\u003e \u003cp\u003e \u003col\u003e \u003cli\u003e \u003cp\u003e \u003cb\u003eMicrostructural Evolution and Dynamic Recrystallization\u003c/b\u003e: Friction stir processing-induced dynamic recrystallization generated refined, predominantly equiaxed grain structures throughout the stir zone in all composite variants. Optical microscopy and quantitative grain size analysis documented progressive grain refinement with increasing graphite content, with the pure graphite sample exhibiting the finest grain dimensions attributable to enhanced Zener pinning effectiveness. Scanning electron microscopy examination confirmed homogeneous reinforcement particle distribution throughout the processed layers without significant clustering or segregation phenomena, even at maximum graphite loading. Energy-dispersive X-ray spectroscopy analysis verified complete particle incorporation and effective metallurgical bonding between reinforcement phases and the copper matrix, establishing that two-pass FSP successfully overcame conventional challenges associated with particle agglomeration in high-reinforcement-density systems.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003e \u003cb\u003eMechanical Property Development and Strengthening Mechanisms\u003c/b\u003e: Vickers microhardness profiling revealed substantial property elevation throughout the stir zone relative to unprocessed copper base material. The pure alumina composite achieved approximately 110\u0026ndash;115 HV through combined contributions from grain refinement and ceramic load-transfer mechanisms. Progressive graphite incorporation produced systematic hardness increases through enhanced Zener pinning of recrystallization fronts, with the 50Al₂O₃-50Gr balanced composition reaching approximately 125 HV\u0026mdash;the second-highest value within the compositional series. Remarkably, the pure graphite composite achieved maximum hardness approaching 140 HV despite the absence of hard ceramic reinforcement, establishing quantitative evidence that fine grain size reduction through graphite particle pinning can equal or exceed mechanical strengthening provided by ceramic particles. Consistent hardness profiles across stir zone widths indicated uniform reinforcement particle distribution and homogeneous strengthening mechanism development throughout the processed layers.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003e \u003cb\u003eTribological Performance and Wear Resistance\u003c/b\u003e: Pin-on-disc wear testing revealed dramatic compositional dependence of tribological behavior. The pure alumina composite demonstrated substantial wear reduction compared to unprocessed copper while maintaining relatively elevated friction coefficients reflecting limited lubrication capability. Progressive graphite incorporation produced systematic improvements in both wear resistance and friction reduction. The 50Al₂O₃-50Gr balanced composition achieved approximately 70% wear rate reduction and 0.45 friction coefficient compared to pure alumina reference values, representing an intermediate composition regime optimizing strength-lubrication balance. The pure graphite composite achieved exceptional tribological performance with approximately 80% wear rate reduction and 0.38 friction coefficient relative to unprocessed copper, demonstrating that self-lubricating mechanisms can provide wear protection equivalent to or exceeding conventional ceramic-reinforcement strategies through fundamentally distinct physical mechanisms. Extended tribological testing over 1000-meter sliding distances documented sustained property maintenance without progressive degradation phenomena, establishing long-term performance sustainability.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003e \u003cb\u003eWear Mechanism Transitions and Surface Interaction Dynamics\u003c/b\u003e: Scanning electron microscopy analysis of wear surfaces documented compositional-dependent transitions in operative wear mechanisms. Pure alumina composites exhibited organized abrasive wear patterns with defined parallel grooves reflecting mechanical particle-surface interaction dominance. Progressive graphite incorporation introduced protective boundary layer formation effects, progressively smoothing wear surface characteristics. Pure graphite composites displayed near-mirror-like wear surface polish with imperceptible wear track features, indicating effective boundary lubrication establishment throughout the testing duration. Wear debris analysis revealed corresponding mechanism transitions from fine angular ceramic fragments through mixed particle populations toward predominantly intact graphite flakes. The observation that minimum-wear-rate samples generated maximum debris particle sizes reflects preferential transfer mechanisms where self-lubricating material concentration at the tribological interface suppresses bulk material removal while establishing continuous protective boundary layers.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003e \u003cb\u003eSynergistic Property Development\u003c/b\u003e: The 50Al₂O₃-50Gr balanced composition demonstrated comprehensive property optimization unattainable by either single-reinforcement system. This intermediate composition achieved near-maximum hardness values, exceptional wear resistance, and low friction coefficients through synergistic mechanisms wherein alumina particles provide mechanical strength and subsurface load-bearing capacity while graphite particles deliver surface lubrication and friction suppression. The superior overall performance of this balanced composition relative to monolithic reinforcement systems establishes that carefully engineered hybrid reinforcement proportions can deliver tribological outcomes exceeding either single-reinforcement strategy, providing quantitative validation of hybrid composite design philosophy.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003e \u003cb\u003eMechanistic Understanding and Structure-Property Relationships\u003c/b\u003e: The investigation establishes clear mechanistic correlations between reinforcement composition and property development. Grain refinement achieved through graphite Zener pinning translates microstructural refinement directly into mechanical property enhancement via established Hall-Petch relationships. Simultaneously, abundant graphite particles facilitate preferential transfer to protective mechanically mixed layer formation, suppressing friction and moderating wear rate acceleration. Two-pass friction stir processing proved highly effective for achieving both objectives, generating refined microstructures without processing defects or incomplete reinforcement incorporation.\u003c/p\u003e \u003c/li\u003e \u003c/ol\u003e \u003c/p\u003e \u003cp\u003eThis study documents that compositional engineering of hybrid reinforcement systems can deliver superior tribological performance through synergistic mechanisms fundamentally distinct from monolithic reinforcement approaches. The demonstration that pure graphite reinforcement can achieve hardness values exceeding ceramic-reinforced reference compositions challenges conventional material design assumptions and establishes self-lubricating mechanisms as viable alternatives to mechanical strengthening strategies for specific applications. For bearing and wear-resistant applications requiring balanced strength and friction suppression, the 50Al₂O₃-50Gr composition provides optimal performance through synergistic property development. For applications prioritizing extreme wear resistance where friction coefficients represent secondary considerations, pure graphite compositions deliver exceptional tribological protection. For applications emphasizing maximum mechanical strength, pure alumina reinforcement provides superior hardness development.\u003c/p\u003e \u003cp\u003eSystematic variation of alumina-to-graphite ratios in friction stir processed copper composites enabled comprehensive evaluation of compositional effects on microstructural evolution, mechanical property development, and tribological performance. The results establish that balanced hybrid reinforcement proportions deliver superior overall performance through synergistic interactions between ceramic strength and graphite lubrication mechanisms, providing quantitative guidance for rational engineering of advanced copper-based composites for demanding bearing, electrical contact, and wear-resistant applications where simultaneous achievement of mechanical efficiency and tribological functionality represents persistent technical challenges.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003eData Availability declaration\u003c/p\u003e\n\u003cp\u003eThe datasets generated and analyzed in this study are available from the corresponding author upon reasonable request. Raw experimental data including scanning electron microscopy images, microhardness measurements, and tribological test results can be provided to reviewers and researchers for verification purposes.\u003c/p\u003e\n\u003cp\u003eDeclaration of interests\u003c/p\u003e\n\u003cp\u003eThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.\u003c/p\u003e\n\u003cp\u003eFunding\u003c/p\u003e\n\u003cp\u003eThe authors did not receive support from any organization for the submitted work.\u003c/p\u003e\n\u003cp\u003eAuthor contributions\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAhmadreza Farjood:\u003c/strong\u003e Conceptualization, Methodology, Investigation, Formal analysis, Data curation, Visualization, Writing \u0026ndash; original draft.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eMostafa Jafarzadegan:\u003c/strong\u003e Conceptualization, Methodology, Resources, Supervision, Validation, Writing \u0026ndash; review \u0026amp; editing.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eReza Taghiabadi:\u003c/strong\u003e Conceptualization, Resources, Supervision, Validation, Writing \u0026ndash; review \u0026amp; editing, Project administration.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eGill, R. S., Samra, P. S. \u0026amp; Kumar, A. Effect of different types of reinforcement on tribological properties of aluminium metal matrix composites (MMCs)\u0026ndash;A review of recent studies. Materials Today: Proceedings, 56: pp. 3094\u0026ndash;3101. (2022).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eRohatgi, P. K. et al. Tribology of metal matrix composites, in Tribology for scientists and engineers: From basics to advanced concepts. Springer. 233\u0026ndash;268. (2013).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eSingh, L., Singh, B. \u0026amp; Saxena, K. K. Manufacturing techniques for metal matrix composites (MMC): an overview. \u003cem\u003eAdv. Mater. Process. Technol.\u003c/em\u003e \u003cb\u003e6\u003c/b\u003e (2), 441\u0026ndash;457 (2020).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eTomar, V. et al. Investigation of mechanical and tribological behaviors of MMC fabricated through FSP. \u003cem\u003eIJERA\u003c/em\u003e \u003cb\u003e2\u003c/b\u003e (1), 1\u0026ndash;15 (2023).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eShajari, Y., Razavi, M. \u0026amp; Nikzad, L. Interfacial investigation of Al/Ni in new generation of composite (Metal-3D lattice metal): Annealing time effect. \u003cem\u003eJ. Mater. Res. Technol.\u003c/em\u003e \u003cb\u003e27\u003c/b\u003e, 3086\u0026ndash;3097 (2023).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eHuang, Y. et al. Dynamic recrystallization and mechanical properties of friction stir processed Mg-Zn-Y-Zr alloys. \u003cem\u003eJ. Mater. Process. Technol.\u003c/em\u003e \u003cb\u003e249\u003c/b\u003e, 331\u0026ndash;338 (2017).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eXie, J. et al. The heterogeneity of dynamic recrystallization and texture evolution in stirring zone of Al-Mg-Sc alloy. \u003cem\u003eMater. Charact.\u003c/em\u003e \u003cb\u003e191\u003c/b\u003e, 112092 (2022).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eBacca, M., Hayhurst, D. R. \u0026amp; McMeeking, R. M. Continuous dynamic recrystallization during severe plastic deformation. \u003cem\u003eMech. Mater.\u003c/em\u003e \u003cb\u003e90\u003c/b\u003e, 148\u0026ndash;156 (2015).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eShehata, F. et al. Preparation and properties of Al2O3 nanoparticle reinforced copper matrix composites by in situ processing. \u003cem\u003eMater. Design\u003c/em\u003e. \u003cb\u003e30\u003c/b\u003e (7), 2756\u0026ndash;2762 (2009).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eOke, S. R. et al. Tensile and corrosion properties of copper and alumina hybrid reinforcement on aluminum matrix composite. \u003cem\u003eJ. Compos. Mater.\u003c/em\u003e \u003cb\u003e59\u003c/b\u003e (13), 1595\u0026ndash;1607 (2025).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eVenkatesh, R. \u0026amp; Rao, V. S. Thermal, corrosion and wear analysis of copper based metal matrix composites reinforced with alumina and graphite. \u003cem\u003eDef. Technol.\u003c/em\u003e \u003cb\u003e14\u003c/b\u003e (4), 346\u0026ndash;355 (2018).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eZhu, J. et al. Differences in mechanical behaviors and characteristics between natural graphite/copper composites and carbon-coated graphite/copper composites. \u003cem\u003eMater. Charact.\u003c/em\u003e \u003cb\u003e162\u003c/b\u003e, 110195 (2020).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eJamwal, A. et al. Microstructural, tribological and compression behaviour of Copper matrix reinforced with Graphite-SiC hybrid composites. \u003cem\u003eMater. Chem. Phys.\u003c/em\u003e \u003cb\u003e251\u003c/b\u003e, 123090 (2020).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eZhou, M. et al. Progress in research on hybrid metal matrix composites. \u003cem\u003eJ. Alloys Compd.\u003c/em\u003e \u003cb\u003e838\u003c/b\u003e, 155274 (2020).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003ePande, S. S. et al. Synergistic Reinforcing Influence of SiC, Al2O3, and CdS on Advanced Tribomechanical Properties of Aluminum Composites. \u003cem\u003eTribology Ind.\u003c/em\u003e, \u003cb\u003e47\u003c/b\u003e(1). (2025).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eSharma, P., Sharma, S. \u0026amp; Khanduja, D. Effect of graphite reinforcement on physical and mechanical properties of aluminum metal matrix composites. \u003cem\u003ePart. Sci. Technol.\u003c/em\u003e \u003cb\u003e34\u003c/b\u003e (1), 17\u0026ndash;22 (2016).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eLiu, Y. et al. Fabrication of graphite-reinforced 6201Al matrix composite with simultaneous enhancement of mechanical and electrical properties by multi-pass friction stir processing. \u003cem\u003eJ. Alloys Compd.\u003c/em\u003e \u003cb\u003e967\u003c/b\u003e, 171835 (2023).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eBaradeswaran, A. \u0026amp; Perumal, A. E. Study on mechanical and wear properties of Al 7075/Al2O3/graphite hybrid composites. \u003cem\u003eCompos. Part. B: Eng.\u003c/em\u003e \u003cb\u003e56\u003c/b\u003e, 464\u0026ndash;471 (2014).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eLi, H. et al. On the tribological behaviors of Cu matrix composites with different Cu-coated graphite content. \u003cem\u003eJ. Mater. Res. Technol.\u003c/em\u003e \u003cb\u003e25\u003c/b\u003e, 83\u0026ndash;94 (2023).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eBaradeswaran, A. \u0026amp; Perumal, A. E. Wear and mechanical characteristics of Al 7075/graphite composites. \u003cem\u003eCompos. Part. B: Eng.\u003c/em\u003e \u003cb\u003e56\u003c/b\u003e, 472\u0026ndash;476 (2014).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eThankachan, T. et al. The effect of friction stir processing on mechanical, wear and corrosion characteristics of Cu-AlN-BN surface composite. \u003cem\u003eHeliyon\u003c/em\u003e, \u003cb\u003e10\u003c/b\u003e(9). (2024).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eOvalı, İ. et al. Mechanical Properties of Al2024/Al2O3/MgO/Graphite composites via hydro-thermal hot pressing route. \u003cem\u003eMater. Chem. Phys.\u003c/em\u003e \u003cb\u003e302\u003c/b\u003e, 127779 (2023).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eChen, Z. et al. Effects of graphite contents on the microstructure evolution, mechanical properties and high temperature tribological behavior of Cu\u0026ndash;Ni\u0026ndash;Al/Gr solid-lubricating composites. \u003cem\u003eTribol. Int.\u003c/em\u003e \u003cb\u003e179\u003c/b\u003e, 108193 (2023).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eSambathkumar, M. et al. Mechanical and corrosion behavior of Al7075 (hybrid) metal matrix composites by two step stir casting process. \u003cem\u003eLatin Am. J. solids Struct.\u003c/em\u003e \u003cb\u003e14\u003c/b\u003e, 243\u0026ndash;255 (2017).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eCartigueyen, S. \u0026amp; Mahadevan, K. Role of friction stir processing on copper and copper based particle reinforced composites-a review. \u003cem\u003eJ. Mater. Sci. Surf. Eng.\u003c/em\u003e \u003cb\u003e2\u003c/b\u003e (2), 133\u0026ndash;145 (2015).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eRamakrishna, N. et al. \u003cem\u003eOptimization of Friction Stir Processed (FSPed) Copper Surface Composite Mechanical Characteristics Using Grey Model (1, N)\u003c/em\u003e Vol. 26, p. 1229 (ES Materials \u0026amp; Manufacturing, 2024).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eHuang, K. \u0026amp; Log\u0026eacute;, R. E. A review of dynamic recrystallization phenomena in metallic materials. \u003cem\u003eMater. Design\u003c/em\u003e. \u003cb\u003e111\u003c/b\u003e, 548\u0026ndash;574 (2016).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eBagheri, B., Abdollahzadeh, A. \u0026amp; Shamsipur, A. A different attempt to analysis friction stir spot welding of AA5083-copper alloys. \u003cem\u003eMater. Sci. Technol.\u003c/em\u003e \u003cb\u003e39\u003c/b\u003e (9), 1083\u0026ndash;1089 (2023).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003ePatel, V. et al. Recent development in friction stir processing as a solid-state grain refinement technique: microstructural evolution and property enhancement. \u003cem\u003eCrit. Rev. Solid State Mater. Sci.\u003c/em\u003e \u003cb\u003e44\u003c/b\u003e (5), 378\u0026ndash;426 (2019).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eZhang, C. et al. Microstructure evolution of thermo-mechanically affected zone in dissimilar AA2024/7075 joint produced by friction stir welding. \u003cem\u003eVacuum\u003c/em\u003e \u003cb\u003e179\u003c/b\u003e, 109515 (2020).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eHirata, T. et al. Influence of friction stir welding parameters on grain size and formability in 5083 aluminum alloy. \u003cem\u003eMater. Sci. Engineering: A\u003c/em\u003e. \u003cb\u003e456\u003c/b\u003e (1\u0026ndash;2), 344\u0026ndash;349 (2007).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eAlmoussawi, M. et al. Segregation of Mn, Si, Al, and oxygen during the friction stir welding of DH36 steel. \u003cem\u003eMetallography Microstruct. Anal.\u003c/em\u003e \u003cb\u003e6\u003c/b\u003e (6), 569\u0026ndash;576 (2017).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eAsadi, P. et al. Effects of SiC particle size and process parameters on the microstructure and hardness of AZ91/SiC composite layer fabricated by FSP. \u003cem\u003eJ. Mater. Eng. Perform.\u003c/em\u003e \u003cb\u003e20\u003c/b\u003e (9), 1554\u0026ndash;1562 (2011).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eZass, K., Mabuwa, S. \u0026amp; Msomi, V. A review on reinforced particles used on the production of FSP composites. Materials Today: Proceedings, 56: pp. 2392\u0026ndash;2397. (2022).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eFaraji, G., Dastani, O., A.A.A., S. \u0026amp; Mousavi Effect of process parameters on microstructure and micro-hardness of AZ91/Al2O3 surface composite produced by FSP. \u003cem\u003eJ. Mater. Eng. Perform.\u003c/em\u003e \u003cb\u003e20\u003c/b\u003e (9), 1583\u0026ndash;1590 (2011).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eDu, D. et al. Modification of the Hall\u0026ndash;Petch equation for friction-stir-processing microstructures of high-nitrogen steel. \u003cem\u003eMater. Sci. Engineering: A\u003c/em\u003e. \u003cb\u003e640\u003c/b\u003e, 190\u0026ndash;194 (2015).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eXue, R. et al. Self-lubrication behavior of the reaction-formed graphite/SiC composites with optimizing graphite content. \u003cem\u003eWear\u003c/em\u003e \u003cb\u003e526\u003c/b\u003e, 204946 (2023).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eSarmadi, H., Kokabi, A. \u0026amp; Reihani, S. S. Friction and wear performance of copper\u0026ndash;graphite surface composites fabricated by friction stir processing (FSP). \u003cem\u003eWear\u003c/em\u003e \u003cb\u003e304\u003c/b\u003e (1\u0026ndash;2), 1\u0026ndash;12 (2013).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eSuresh, M. \u0026amp; Mayakannan, S. Optimization of wear behavior in Ti-6Al-4V hybrid nanocomposites reinforced with TiC, ZrO2, and graphite using friction stir processing. (2025).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eVaira Vignesh, R., Padmanaban, R. \u0026amp; Datta, M. Influence of FSP on the microstructure, microhardness, intergranular corrosion susceptibility and wear resistance of AA5083 alloy. \u003cem\u003eTribology-Materials Surf. Interfaces\u003c/em\u003e. \u003cb\u003e12\u003c/b\u003e (3), 157\u0026ndash;169 (2018).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eShajari, Y., Nikzad, L. \u0026amp; Razavi, M. Investigation on the effect of strut thickness and heat treatment on the mechanical energetic properties of open cell Ni-foam during a uniaxial compression. \u003cem\u003eJ. Mater. Res. Technol.\u003c/em\u003e \u003cb\u003e36\u003c/b\u003e, 1802\u0026ndash;1812 (2025).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eXian, G. \u0026amp; Zhang, Z. Sliding wear of polyetherimide matrix composites: II. Influence of graphite flakes. \u003cem\u003eWear\u003c/em\u003e \u003cb\u003e258\u003c/b\u003e (5\u0026ndash;6), 783\u0026ndash;788 (2005).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eRanjbar, M. et al. Investigating the effect of optimal addition of Inconel 718 machining swarfs on the wear behavior of gray cast iron. \u003cem\u003eJ. Mater. Res. Technol.\u003c/em\u003e \u003cb\u003e35\u003c/b\u003e, 2558\u0026ndash;2572 (2025).\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"Friction stir processing, Hybrid composites, Alumina-graphite reinforcement, Tribological performance, Microstructural refinement, Wear resistance","lastPublishedDoi":"10.21203/rs.3.rs-8488460/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8488460/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eHybrid metal matrix composites of copper reinforced with varying proportions of alumina and graphite particles were fabricated through two-pass friction stir processing. Five compositions spanning 100% Al₂O₃ to 100% Gr were investigated through microstructural, mechanical, and tribological characterization. Dynamic recrystallization generated refined equiaxed grain structures with progressive refinement as graphite content increased, attributed to enhanced Zener pinning. Vickers hardness measurements revealed compositional dependence, with pure graphite achieving maximum hardness of approximately 140 HV\u0026mdash;surpassing pure alumina\u0026mdash;demonstrating that grain refinement through self-lubricating particle pinning equals ceramic-based strengthening. Pin-on-disc testing documented 80% wear reduction in pure graphite relative to unprocessed copper. The 50Al₂O₃-50Gr balanced composition achieved optimal performance through synergistic effects: near-maximum hardness, 70% wear reduction, and significantly reduced friction coefficients. Wear mechanism analysis established progressive transitions from abrasive wear in alumina samples toward boundary lubrication in graphite-enriched materials, confirmed through wear surface morphology and debris characterization. The investigation demonstrates that carefully engineered hybrid reinforcement proportions deliver superior tribological performance through synergistic interactions between ceramic strengthening and graphite lubrication, providing guidance for designing advanced copper-based composites for bearing and wear-resistant applications.\u003c/p\u003e","manuscriptTitle":"Synergistic Effects of Alumina and Graphite Reinforcement on Microstructural Evolution and Tribological Performance of Friction Stir Processed Copper based Composites","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-01-09 07:12:53","doi":"10.21203/rs.3.rs-8488460/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2026-02-02T18:35:03+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-01-31T17:50:38+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-01-19T06:02:36+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"142357579116244582096269341372069771243","date":"2026-01-10T04:19:57+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"24158710454792765164406425678798376002","date":"2026-01-09T12:01:41+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"38554247770023001895387698823132472988","date":"2026-01-09T09:33:21+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"255795412765971498695478133337187439045","date":"2026-01-08T07:34:14+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2026-01-07T10:15:49+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2026-01-07T04:13:30+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"","date":"2026-01-06T18:55:33+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2026-01-05T06:16:17+00:00","index":"","fulltext":""},{"type":"submitted","content":"Scientific Reports","date":"2026-01-05T06:08:21+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"a4ef09b4-6eda-4729-941f-c96fafeb5b7b","owner":[],"postedDate":"January 9th, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[{"id":60794420,"name":"Physical sciences/Engineering"},{"id":60794421,"name":"Physical sciences/Materials science"}],"tags":[],"updatedAt":"2026-03-02T10:40:59+00:00","versionOfRecord":[],"versionCreatedAt":"2026-01-09 07:12:53","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8488460","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8488460","identity":"rs-8488460","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}